When I first turned away from religion, it was not under neutral circumstances. I felt religious people were hostile towards me (and all of humanity), and I was in turn hostile towards the religious. That changed over time.
For one thing, there is no massive conspiracy to turn everyone Christian or Muslim. A conspiracy is, after all, planned in secret. These religions are quite open about their intention to convert others. Religious believers are not trying to be intrusive or annoying, it’s just the way their ideology is constructed. One has to remember (or if one was never religious, imagine) the mindset of the believer:
Imagine you see a group of children playing in the street, and a car is coming. You probably wouldn’t approach them politely and ask them in a reasonable manner to kindly move themselves out of the way. No, you would probably shout frantically, maybe even run up to them and push them out of the way. Christians really believe they are saving people’s souls… literally. Since the stakes are so high (eternal salvation versus eternal damnation), things can get very intense.
It is quite easy for Christians to maintain this sense of urgency regarding faith. Those who are this dire hold dear the command to “Watch therefore: for ye know not what hour your Lord doth come” (Matthew 24:42), for they “know not when the master of the house cometh” (Mark 13:35). In the minds of evangelical Christians, there is an impending doom hanging over all of humanity, and only Jesus (with an assist from the believer) can save us. The problem is, it all comes off as rather rude and alarmist.
This is partly due to the Bible’s shoddy morality. The primary tenant of Christian ethics regarding the treatment of others is the “Golden Rule:” do unto others as you would have done unto you (Leviticus 19:18, Matthew 7:12, Luke 6:31, Luke 10:27). This is a pretty good rule, but it has lots of major holes. This does not account for the fact that your neighbors almost certainly have completely different ideas of how they want to be treated.
This is the root problem. Many Christians think, “If I were not Christian, I would want someone to introduce me to Christ.” Except… they wouldn’t. Christians don’t like other religions trying to recruit them, so why would a Christian think their neighbor wants to have Christianity shoved in their face? Would Christians tolerate Buddhists knocking on their door at 10am and sharing their love of meditation? How about Hindus outside McDonald’s carrying signs depicting a slaughterhouse floor? Just look at the holy hell they rose over a few atheist billboards.
It is futile to try to discourage proselytizing. The Bible is clearly and unequivocally in favor of it, as it is full of calls to “spread the word.” In the end, you can’t really justify getting angry at Christians. Blame the faith, not the faithful. Forgive them, for they know not what they do.
Christianity is just inherently invasive. Most Christians choose to show a fair amount of restraint, but they are doing so against the calling of their faith. In a way, Christians who keep their faith to themselves are not good Christians, just good people. Lucky for us, most people make rotten Christians.