(The following theories are mostly stuff I thought up in my more paranoid moments. Aside from evidence, they also often lack internal logic and consistency, so it should, by all experience, be possible to get a lot of people to passionately believe in them.)
1. The Mayan prophecy was actually correct and the world did end on December 21, 2012. What we are currently living in is purgatory.
2. You don’t know that many people. You may think that you are constantly meeting new folks, but actually, it’s all just a small band of incredibly versatile actors (kind of like the movie “The Truman Show” on steroids). The reason that both that girl you met on vacation and one of your university professors occasionally remind you of your mother is that they are, in fact, all the same person. “What if you got all of them to gather in the same room?”, you may foolishly ask. Well, ever heard of camera tricks?
3. You are never going to die. Instead, there is an infinity of parallel universes, and each of them, for every event that would kill you, splits up into one world where the event happened and some other world where the event is prevented by some crazy coincidence. Normally, when history branches in two like that, each of the new parallel universes would contain a version of you that thinks of herself as continuing your life, but if in one universe, you have ceased to exist, no version of you can have the feeling of living on in the new universe. Thus, your subjective experience is that you always continue your existence in some universe and, hence, never die, even as you eventually watch all people you ever knew die (who, of course, will also subjectively live eternal lives of their own).
(Like all theories on this list, I believe this one is a stupid idea, but if I ever celebrate my 1000st birthday, I will actually start to get suspicious.)
4. “Game of Thrones” is actually set in the prehistoric Marvel comics universe, and the Starks of Winterfell are ancestors of Iron Man. Because he is Tony Stark, remember? Also, Atlantis exists in the Marvel comics and is supposed to have been the size of modern Australia until it sank into the ocean 21,000 years ago, so maybe it’s identical to Westeros. (This of course assumes that there will be any surviving Starks left by whatever time GoT ends.)
5. Procrastinating on important tasks is not what it seems. Actually, you constantly get involved in secret missions to save the world from serious global threats (pandemics, zombies, aliens, or whatever). Afterwards, your memories of all that happened are wiped and replaced by recollections of you wasting time in front of a computer. So if you ever failed an exam, it’s because you put the greater good of humanity above your own selfish desires.
6. People constantly go back in time and repeat some period of their life again. They just don’t usually notice, as these “periods” are extremely short, typically only a few nanoseconds.
7. Earth is a sphere, but we live on the inside. This so-called “hollow earth theory” actually isn’t original to me.