If you've got time, here's a theory I found on the Lostpedia forums that actually makes sense to me:
The assumption is that we (as we sit in front of our computers right now) are the co-creators of a dream called "reality".
We are co-dreamers and The Light is the "manifestator" of our thoughts.
The light manifests wishes (desires) but unfortunately also anti-wishes (fears, barriers). You could actually claim that The Light magnifies _problems_ because a problem per definition is a "wish PLUS barrier".
Now:
The Mom said that every human has a little bit of the Light (= the power to make one's wishes reality or be one's own enemy). Basically we could reword her as "Each person shapes his or her own fate!" or "Every man is the architect of his own fortune."
She also said in this context "people always want more": The more you see that your wishes get fulfilled the more wishes you want fulfilled.
That also explains why everything will SEIZE to exist: If The Light that constantly manifests these wishes (of us all) will go out then the world as we know it (= the sum of all our wishes) will disappear.
The closer you get to the light the stronger the manifestation. Thus the temple water could heal immediately (this was the hand cutting test of Dogen).
Thus the crashers could survive: Their life wish got fulfilled as soon as they came close enough to the island. Others who died in the crash made their prayer, let go, found peace in the seconds before the crash and thus died. Our Losties survived because they were the most troubled ones. They didn't let go because they had too big personal issues (as shown by the flashbacks) to be left unsolved thus they wanted to live (instead of making peace with their lives).
(If you don't like this reason why they survived then speculate that Jacob's wish saved them.)
Examples of manifestations of wishes or fears fulfilled:
Michael doesn't want to lose Walt? Walt gets kidnapped.
Claire is afraid to be a mom: Kate takes away her child.
MIB hates the island, wants to be special, wants to work for noone, wants to leave and that's what he got: Being special (smoke), being a hater, having his greatest fear fulfilled: Being summonable and being stuck (which was actually a combination of his wishes and his mother's wishes)
Additionally it could be that MIB fears water (since Jacob quasi-drowned him) thus this fear also gets manifested.
Ben cares only about his daughter: Daughter gets killed.
Jacob's cabin? Ben's own manifestation of Jacob and his fear of Jacob and his fear that Jacob might give his power to someone else ("Help me").
Hurley's fear of being the Jacob's replacement: Hurley becomes the protector
Jacob thought this island was too wonderful to leave: So his wish resulted in the release of the smoke monster who made this island less wonderful.
Nearly all Losties have issues with being unwanted/accepted: They become the candidates/saviours (= their wish to be wanted/important is fulfilled) but with much pain.
Dharma wants to drill: Dharma never finishes drilling.
Zoe likes it to get on everybody's nerves: Someone shut her up.
When Jacob touches someone outside of the island they get somehow a closer connection to The Light thus there is an attraction + alignment. That's why Hurley had the numbers in his head (or that's why Hurley's numbers manifested on the island).
EVERYTHING THAT YOU TRY TO _FIGHT_ (instead of "letting go") will lead to more fighting/destruction (they come, corrupt, destroy). Because of the constant manifestation of wish and anti-wish.
The only solution is _to let go_ (= let go of one's desires and fears).
That's why Michael is stuck: Because he is "terribly sorry". As long as he is "Terribly sorry" he cannot leave. The whispers represent BROODING of the dead.
Depending on the circumstances "Letting go" can be translated as "the island was done with them".
If you try to SOLVE (instead of letting go) then solutions become problems because The Light magnifies wishes and their accompanying fears (= the fear to fail etc). Thus "Every question leads to another question" or Every solution leads to another problem.
This is why Jacob never said CLEARLY what to do. Or HOW to do it. Because that would lead to Jacob-solutions that lead to Jacob-problems which are not "letting-go'able" by the person who performs the Jacob-solution. You can only let go the problem that you yourself created.
That's why this island is full of uncompleted tasks, projects, unfulfilled desires and open questions: Dharma, military, temple, baby crisis etc... The accumulation of wishes, solutions, backlashes, new problems, solutions to the new problems that backlash, fighting against counter-wishes etc. The bigger the solution the bigger the backlash.
That's why Smokie obides by the rules of The Mom (to not kill Jacob) and Jacob (to not kill Candidates). Because fighting against protectors' wishes is of course like fighting against the uber-wishes of the island. It would backlash huge time and lead to far greater problems. Thus MIB goes by the Judo-strategy (using the opponent's own energy to throw him on the ground): He wants to cause a collapse within the system WITHOUT making NEW wishes or fighting OLD wishes.
"Now you and I are the same" means that now we _CAN_ know the wishes of all other around: If you know your own wishes and the wishes of others (= you can see the world through the eyes of others, you can see the united collective direction of plans of others) then you are somewhat interwoven into the desires and fears of others and the island. That's why they are the same: "We see other people, We feel other people, We think other people, We feel each other".
The others: They try to be as counter-wish-free as possible: Little emotions, no regrets, coolness, free from counter-wishes, trained to let go of bondings and emotions. They kill their own parents and demand of John Locke to kill his father. This is also why Ben didn't want Karl and Alex to be emotionally attached to each other.
Room 23? It's a training to erase emotions, counter-wishes and to become able to "wish at will without fears":
"Plant a good seed and you will joyfully gather fruit" = be careful what you wish for since it could include your current fears
"EVERYTHING CHANGES" = Be careful what you wish for since you could get stuck with it even if it becomes worthless in a few decades/centuries
"We are the causes of our own suffering" = dito
"The extinction of desire is nirwana" = dito
"Divest your ordinary nature and find your true nature" = dito
"God loves you as he loved Jacob" = This one of the mysteries in the bible because god loves jacob (and hates esau) without them having done anything specifically good or bad ("the LORD says 'Yet I have loved Jacob, but Esau I have hated'") = Do your acts, don't ask whether it will lead to approval or not. Don't desire asking "Why" or "Why me?"
"Overdoing things cannot lead to happiness" = investing too much emotion is bad
The psychedelic movie features yin-yang (wish + counter-wish), desirable things, disturbing things etc
The movie is supposed to make you _cool at will_.
In other words: A possible way how to overcome the wish-counterwish-dilemma is to completely shut down any emotions (e.g. fears, twinges). This is what happened to Sayid and this is why MIB used Sayid and Ben (= one of the others) to put the work into practice. This is the loophole (both in the sense that MIB technically didn't kill Jacob and that Ben was counter-wish-free enough to do the task).
This is also why Widmore and Eloise (both are Others) are so influential. They got successful with their no-counterwish-technique and their connection to the island.
You can also understand why Ben tried in the last moment to DENOUNCE Alex stating she meant nothing to him and why Widmore stated it was Ben's own fault that she got killed. This is also why Mom didn't name MiB: Inventing a name is already too personal and too bonding.
Jacob's cabin seems to manifest fears (eyes, voices etc) thus you'd know if someone visited it (by the newly manifested inventory, like jars, pictures etc). Claire being in the cabin without fear shows that she had already the Sayid infection (= could not feel anything).
What happened during the Juliet's "bomb detonation"? The Light was released and their wishes united and created an alternative reality:
This alternative reality is like our CURRENT reality except that it is co-made not by all of us (like our current reality) but only co-made by the Losties.
The wishes were
1) to make the plane never crash
2) to forget this goddam island
They did not change the past since WHH (what-happened-happened) applies. Instead they created an alternative reality in the future (= after they die) and got stuck in it because the wish to reverse time is a choice to NOT move on.
But let me repeat again the major plot of the island (the big picture of the script):
Everything that is created on the island will get destroyed (completely or partially).
Tawaret, The Swan, Dharma, Danielle's science expedition, the Temple people, the ship Black Rock, Smokey, the Others, the Candidates, Jack, John....
Anything that MATTERS will get ANTI-MATTER'ed, so to speak.
Thus the question "How was it built/How did it come here?" is actually irrelevant. The whole island is "a coming and going" and only the Light stays. Probably there were visitors before the Egyptians and there will be after Hurley.