I can think of at least four reasons why a Christian may wish to debate an atheist:
- 1. They wish to understand why some people do not believe in any gods including the Christian god.
2. They want to convert atheists to their brand of Christianity.
3. They're hoping to help other Christians maintain their faith by making the atheist look foolish and the atheist's doubt appear to be unreasonable.
4. They struggle with their own doubts and look to the atheist to offer hope that Christian claims may be true.
The corollary of this phenomenon is also an important factor in debates between atheists and Christians. If no agreement from the atheist is forthcoming, then some Christians may become upset. They might see the atheist as angry or hateful because he doesn't give them what they want so badly: assurance that they may get to heaven after all.