One part of the answer to your question is that not all of them do. Some do, though, so let’s examine the range of motivations that might explain it. The “range” includes emotional, religious, social, and political reasons, and also the range from trivial and stupid, to important and profound. So, we have a very large subject ahead of us.
One hypothetical reason:
I believe in my religion, it seems to me to be obvious truth, and I feel that not believing the same thing can only be the result of some absence of information. So, I tell everyone I meet what I believe, thinking that they will flock to join my church, and be better people because of it. This is a pretty arrogant point of view, but I certainly think it does represent one set of motives, although it is quite simplified.
Another one:
I believe my religion is the only truth, and all other points of view are heretical, and evil. Proselytizing is therefore a holy duty, and all tactics are justified, since it is the defense of good itself that requires it. With this sort of motivation, all the extremes of human misery are justified, and eventually encouraged.
Another one:
I flatly don’t believe, but I find that passionate faith among others makes it easier for me to accomplish my political goals, so I pretend passion, and faith, and use the faithful as pawns to increase my power. Hook this one up with the last one, and you have a possible political dictatorship with unlimited power.
Another one:
The secular world, and the secularly oriented people I have known have hurt me, and exploited me, and made me feel small, and unimportant. Then my co-religionists changed my life, made me feel loved, and important, and able to avoid being exploited. I am now strongly motivated to do the same for people I meet that seem also to be hurt, or weak, or exploited.
Another one:
God really did speak to me, and He told me to give you this message.
Another one:
The belief and faith that I have found has given me insight, and understanding of human nature, and I have applied that to my own behavior. That behavior is (as far as I am able to be consistent and faithful to my beliefs) an example of the sort of life that benefits humanity in general, and the people I meet in particular. I don’t try to convince you, since logic and rational thinking are not a part of my faith. I tell you what I believe, only when you ask, because that is the only time you will actually hear what I am saying. The rest of the time, I treat you as my beliefs require me to, because I believe that to be the right thing to do.
Another one:
All the rich and influential people in my town are members of one church, and part of that church’s expectations are that I will proselytize publicly, and be seen as an active recruiter for my church. Since I really want to have access to all these influential people, I preach my ass off, every time someone fails to run away fast enough.
I could go on, of course, but there isn’t much point. You cannot tell which one is my real reason for proselytizing, since you can’t see into my heart. You can judge by the life I live, and the way I act, what my real values are, but to do that, you have to get close to me, and come to know my real self. A bit much to expect you to do with every evangelist you ever meet.
Tris
“When people are free to do as they please, they usually imitate each other.” ~ Eric Hoffer ~