Egotism has a lot to do with it. When you belong to a religion you belong to the "in crowd," the "saved", the "redeemed," etc.
But simple acceptance of cultural norms has a lot to do with it as well. People believe as they are taught to believe, which is why theists are usually against allowing their children to be exposed to other "undesireable" beliefs.
I agree with you Brian. Your last statement, saying that expecting a reward for what you have done is a contradiction to what the bible says. Seems that there arent many things from the bible that dont have a contradiction.
Social acceptance is another reason why i think most believers hold on to religion. Like most of these stories I've been hearing about upcoming High School graduates mentioning the prayers being said during the ceremony and being outcasted because they are a non-believer.
It gives them a feeling of security, is one main reason. I'm not sure why that is, since the Bible says God makes it to rain on the rich as well as the poor. I think they may just get a general feeling of "I'm doing the right thing." I think it may have a lot to do with childhood indoctrination.