To better understand people of differing beliefs, you have to temporarily pretend to accept their fundamental tenets.
To use your example, understand that most sects of Christianity teach that in order to gain entrance to heaven, one must accept Jesus as one's savior and at least try to follow the moral code promulgated by Christianity. People who do not will go to hell and suffer an eternity of torment. Understand, this is not a theoretical "what if" to them, they believe this to be true. Completely.
Thus, if you are a good person, would you not want to save others from eternal damnation? I mean, what kind of heartless human being would just let others go to hell without even trying a little? A good person, a kind person would therefore try to convert others to the path that would lead them to heaven.
Or similarly, if you believe a fetus is a human being then abortion is murder. Not the "termination of a fetus", but the literal equivalent of chopping up a newborn baby. If you understand that some people actually believe this, it is easy to understand why most pro-life arguments just seem facile to them. To them, those arguments are simply attempting to justifying the literal murder of innocent babies.
Putting yourself into the shoes of others and clearly understanding (not accepting as true, just accepting that they believe it is true) their underlying reasons is, IMO, critical to understanding what may, to you, be "crazy" or even "evil" positions.