Q: Why do atheists have to ruin it for everyone else?
A: I think what is usually meant by this question is something like "I'm happy believing what I believe, why can't you just leave me alone in my belief instead of trying to question it?"
Most atheists would, I suspect, be perfectly happy to live and let live. If you're happy with your faith system, good for you!
Unfortunately, atheists are rarely afforded the same luxury. The majority of Americans are people of faith and churchgoers, and trust atheists less than terrorists. That tends to produce an environment where those who are different (i.e. atheists) are looked at with suspicion. When an atheist speaks up and makes it known that he or she is not a believer, this usually results in a fair amount of attempted proselytizing and questioning. When the atheist responds, often the reaction is hostile.
Apparently we're supposed to just shut up and take it.
Of course, having said that, there are atheists who are much more vocal and active about expressing not only their view of reality, but who wish to force theists to confront the challenges to their faith head-on. The reasons for this are manifold, but at the root of all of them is, I suspect, the very human desire to not only be right, but to have others acknowledge that we are right.
At the end of the day, however, most atheists are happy to let you go on believing whatever you like, so long as they are afforded the same opportunity.