Why do you believe that your body isn't all there is to you and that when it ceases to exist, along with everything else in the universe, you won't cease to exist, forever? A feeling? Faith? Wishful thinking?
Don't know. Just do... Just like I believe that the Lakers winning an NBA championship makes the world slightly crappier.
I guess that faith is the word for it, as faith is, by definition, belief despite the absence of any evidence. Either way, though, whether it's belief in nothing, or belief in something, it's still belief, and devoid of evidence supporting either theory, atheism is as much a faith as structured religion. Usually, in my experience at least, Athiests have more faith in their beliefs than the religious do in theirs. To me, any group aggressively professing total certainty in something they cannot possibly know is just that--aggressively professing total certainty in something they cannot possible know. Atheists and religious fanatics alike.
No level of personal conviction, stubborness or certainty in any view regarding the greater meaning of things will ever raise that conviction to anything beyond faith.
So in short, yes, it must be faith, because I can never be certain. But it works just fine for me.