Why are people religious? Why are you religious?

Hello. I'm curious as to why people are religious. I myself am not religious. To be perfectly honest I could careless if there is or is not a God... I just really don't care one way or another. But what fascinates me about religion is how people can get so worked up over something I consider so unimportant.

What is it that causes someone to be religious and someone like myself to not care at all? How can people get so passionate over something so unknowable?

As I said I don't really care about religion or the afterlife but I personally cannot understand why some people care so much. What's the big deal?