I'm Roman Catholic.
Christians spreading their religion has a lot to do with the belief that the only way to salvation is through Christ. Through this belief comes the effort to spread Christianity in order to maximize the number of people who are saved.
That and if we convert enough people, we get a free toaster oven!
j/k
In other news, I am Roman Catholic right now mainly because it was the way I was brought up. I am currently in the position of questioning my faith. While I am still a practicing Catholic and I do attend church every Sunday, I am going through the process of discovering why i believe what I do and how I can make my faith more a part of my being instead of just something I do.