Pulse with Haruhiko Katô and Koyuki. This is the original Japanese version and not the American remake. This film is definitely one of those perplexing, sit back and think about it kind of films. It really wasn't as scary as we had figured, at least not in the things jumping out at you and blood and gore kind of way. It's one of those subtle and creepy scary movies.
SPOILERS AHEAD!
Now, one thing I do know for certain regarding this movie is that the Japanese must have a terrible fear of being alone, this is evidenced by this film and also Suicide Club. The thing is this loneliness they are so afraid of is one of those quasi-literal things. In the movie they keep saying how alone they are despite the fact that they do have people around them. But there are no extras, there are just the main characters. With that in mind I figured they meant they were alone in that no one understood them or cared about them since the people didn't seem to really bond with each other and were more linked through a mutual fear. So there's that part, but then is seems that the ghosts, who were overflowing from the realm of the dead (why our Universe is infinite but theirs is very finite is beyond me, but whatever) compel people to kill themselves by making them think they are not alone but by suicide the people doom themselves to be alone forever.
But why do they do that? They (kind of) claimed they wanted to trap other people in their own loneliness and the goal there seems to be that keeps the newly dead from coming into the already overcrowded realm of the dead and that it's a matter of self preservation (I think?). But the guy that died in the beginning ends up compelling his friends to kill themselves, it seemed like he was tricked into the same kind of thing he ended up tricking the others into, so he wasn't going to that realm of the dead anyway. So why did he do that? I don't think that was his motive and I'm guessing there was something greater to this than just: once you die you become a ghost, ghosts are evil, the end.
The only thing I can think of is that the ghosts can only be not alone when they're in their realm, so the ones on Earth are forever alone, they can't see or interact with other ghosts (I don't think) so they think they're the only ones. The ghosts then try to bring the living into their own microcosm, which of course doesn't work and only begets more lonely ghosts. This process continues over and over again until at the end of the movie there's no one left in the world.