For Christians, proselytizing is a part of their religion. Why do atheists object so much when Christians proselytize to them? If they simply don't believe in God, why care that Christians choose to share their beliefs?
Proselytization is central to the religious faith of many evangelical Christians — they will "share" their beliefs with anyone they come across if they are given half a chance. They believe they are doing a good deed, but from the other side it often doesn't appear very positive. This can be difficult for many Christians to understand because they accept without question the truth of their religion and are thus convinced that there is nothing wrong with sharing this truth with others.
The primary problem doesn't lie simply in disagreeing with the Christian message, as most targets of evangelism do, but rather in the general attitude that lies behind the evangelism itself. Although Christians may profess to be doing a good deed by sharing their religion with others, in reality it's very often the case that they are simply not treating non-believers with the respect and consideration they deserve. In Rapture Culture: Left Behind in Evangelical America, Amy Johnson Frykholm writes:
[F]or many of the people I interviewed, witnessing is an essential part of their Christian faith. In order to understand that faith, I had to be willing persist in listening even when my salvation, beliefs, and life became the uncomfortable object of the conversation.
I am using the word object here deliberately because, in many ways, I found witnessing to be an objectifying experience. No matter in what way I articulated or failed to articulate a reasonable position for myself, my lack of belief turned me into an object. In Martin Buber’s language, I often felt at these moments that I turned from a “Thou” in the conversation into an “It.”
Few evangelicals understand how they come off to those they are attempting to evangelize; fewer still appear to care. This would seem to be a contradictory attitude: they act like they evangelize because they care, but if they really cared then they would not want to even appear to be treating others like objects. Even if we assume that evangelism isn't necessarily objectifying, it still becomes objectifying far too often. This is something which should disturb Christians as much as it does those who are targeted for proselytization — after all, it's their attempt at a good deed which is undermining their own goals.
The apparent contradiction seems to be resolved because evangelical Christians imagine themselves to be superior to others in the sense of having access to privileged information. They adopt a paternalistic attitude: their actions may be perceived in a negative fashion, but because they know so much better than others, this is a price wroth paying in order to rescue people from their own sinfulness. How often do parents do things perceived by their children as unfair, mean, or arbitrary but which parents know to be in their children's best interests?
This, however, is no better than treating a person like an object in the first place. An attitude of self-righteous superiority is no way to show to others that one does know something important and helpful. Evangelical Christians may be sincere in their faith and even sincere in their desire to help others find their god's love, but they are not sincere in their respect for others so long as they continue to objectify people as part of the process of evangelization.
Unfortunately, such objectifying experiences tend to be one of the primary ways in which atheists have to interact with Christians when it comes to the context of religion and theism. Obviously atheists interact with Christians in stores and businesses all the time without being objectified, but in conversations about religion and theism such objectification is far too common. Christians write posts to atheists online with the sole purpose of proselytizing. Atheists are treated not as human beings worthy of equal consideration, but as targets — almost as if they are being hunted. It shouldn't be a surprise when atheists find this annoying and complain.