Sorry for your loss, man. Very similar thing happened to a friend of mine, he lost his little sister. Things are just like that sometimes, and since we know nothing about death, we're scared, saddened and depressed. It's natural. I wouldn't worry though. Just because we don't understand death doesn't mean it isn't natural. It's as natural as living, we just can't comprehend it. Crazy idea, when you think about it.
What surprises me so much is that people really think we can know about our idea of god, death, an afterlife and such. The whole thing is, god is an abstract human idea, conjured by our brains, and whether or not it's true we simply cannot know. Maybe we'll find a way to get into this kind of stuff some day, but right now, nobody can know whether or not there is a god, gods, or anything like that which is beyond our perception. Not much point in trying to apply logic to this one, in my opinion. If you want to believe stuff, by all means do, since belief is the whole point.
I don't really subscribe to any particular theory. Some make much more sense than others, as some people think some pretty hairbrained stuff, but within the bounds of reason, no one person's beliefs are any more or less valid than anyone else's. I'm basically in a state of curiosity about where we come from, where we go, what happens to us, and so forth. I'm curious and open, but largely it's stuff we can't really know, so there's not much point beating ourselves up about it. You have to wonder if some of this will be more apparent once we die, but I kind of think not. It makes sense to me that we'd be recycled in some way. If energy cannot be created or destroyed but rather exists always in some form or other in the same quantities as always... if matter can't be created or destroyed, but always exists in some form somewhere.. it would only be a logical progression of thought to assume our "life energy" or "souls," or however we put it, are in some way reused in different forms. But yeah, I know, I said no point in using logic, and I'll hold to it since we can't really know what's up with the world. Makes life kind of interesting, and it really pisses me off at people who go kill other people because they think their opinions are more legitimate.