Most likely, there's more than one reason why believers often enough condemn what others believe and try to convert others to their own beliefs. Offhand, I can think of three possible reasons.
Human Instinct. A number of scientists are of the opinion that our preference for sharing the same beliefs as others is an instinct that evolved in us as a means of furthering group cohesion, which would have been advantageous to our survival.
Religions and Social Stability. From the time of the earliest civilizations, a clear purpose of many religions has been to further social stability by providing an ideological justification for the existing social order. You can see this through-out history. The effect has been that to question the religion is almost to question the existing social order -- at different times and places, doing so was treated in much the same way as an act of treason would be treated. Even today, there are plenty of people here in America who associate atheism with being anti-American.
Inadequate Education. Human instinct and peer pressure to conform are not easily overcome, but a good education goes a long ways in helping to balance those things out. Unfortuntely, not a lot of people these days have a good education - especially in America. What they have are educations suited to making them employable. But there is a huge difference between being employable, and being well educated.
