Why Do Atheists Argue?
A good response to the question can be found here.
I argue because I care about people, because it saddens me to see people frittering away a good portion of their very short time on earth in pursuit of pointless things, like worshipping and praying to a malicious supernatural entity, e.g. the Abrahamic God. Think of how many things you could otherwise be using that time for, things that would actually be helping to make this a better world to live in.
The entire article is a good read, I suggest everyone go there and take a look at it.