[The following post is an opinion. The author claims no omniscience, but believes the following to be substantially true.
Of course, I could be wrong. :D]
The problem with “evangelizing”, whether it’s Christianity, Judaism, Islam, Buddism, etc., is this: People tend to approach the problem of informing others of their beliefs like they do any other situation where they feel they need to persuade someone else. In some cases, they don’t really believe it themselves, and can only validate their (shaky) choice to believe by convincing others. Hey, If I can convince you, then maybe I’m right after all?
Others just don’t understand how someone could possibly disagree with them. They don’t seem to understand that you’re not clairvoyant, and therefore don’t have the same understanding of the subject you do. You’ve met people like this, haven’t you? Even outside the context of evangelism – they’re the people who believe you’re disagreeing with them for the simple pleasure of watching them get steamed. Either that, or you’ve already signed with the opposition and therefore your position must be brutally dissected and debunked, just so any onlookers can be sure that you are wrong, and they are right. These are the folks that treat any form of disagreement with them as a personal insult.
Yes, it’s dysfunctional, but hey, a lot of folks go into “helping carreers” (medicine, social work, psychology, etc.) because at some level they want to “fix” everything, including you. Why shouldn’t a vocation (or avocation, in the case of non-ordained evangelists) that includes “helping people find G-d” attract some of these "fixers?
Then, of course, there’s the folks who don’t really believe, but either figure it (religion) is a good, legal way to separate a fool from his money and gain instant respect and credibility (within the group, anyway), or feel that, if religion is a crock, it’s a crock that helps others, even if it doesn’t help them, so why not help perpetuate it and get some of that “feel good” second-hand, by helping others to feel good, poor deluded souls that they are?
Such behavior is not confined to the “really, really religious” folks. Stop for a moment, and I’m sure you can remember some folks who’ve been obnoxiously “righteous” about such non-religious topics as the truthfulness of some urban legend, capitalism v.s. communism, or gun rights v.s. gun control.
I’m a Christian, but I figure that G-d chooses whom to call, and it’s an individual’s choice whether they are willing to listen. My witness is my life and how I try to live it (those of you who know me, know it isn’t always a bed of roses :D). If you want to ask me about something related to my religion, go ahead, but remember that I believe “I don’t know” is a good, honest answer that does not not necessarily invalidate my beliefs. If you don’t ask, I won’t try to buttonhole you and MAKE YOU LISTEN YOU NO-GOOD DURTY HEATHEN! 
It isn’t my job to herd people into the kingdom of heaven. Neither is it my responsibility to bar the way of others seeking entry. I just try to ensure I stand in the right line, and don’t step on any toes (unintentionally – some toes need treading upon ;)). I believe if you really seek truth, you’ll find just as much as you can handle. If that’s not what you’re looking for, you’ll find something else. If you’re seeking justification to continue doing things the way you want to, you’ll probably find that. If you’re looking for ways to control others (or abdicate responsibility for your own decisions by allowing others to control you), then that’s what you’ll find.
The truth never hurts anyone, but it sure irks the hell out of some folks.
~~Baloo