While I frankly didn't appreciate the question very much, on reflection I realized it was a fair one.
The short answer is my will to live. As Lisa Costello has said, if will to live were sufficient for survival, I'd live forever. I can imagine letting go in the very late stages of my terminal decline, if I were overwhelmed by the physical and psychic pain of dying, but not short of that.
The slightly longer answer is that
The more complex answer, as simple as it may be on the surface, is my atheism. Despite thousands of years of wishful thinking and uncountable faith narratives from virtually every human culture, there is not one shred of objective, repeatable evidence for the survival of self beyond the death of the brain. When I die, I will experience personal extinction. That's not a belief, that's not a theory; that's a simple, empirical fact borne out by the experience of every human being who has ever lived and died before me. While I'd love to be an exception, given that basic truth of course I want to hang around the party as long as possible.
One final point: once I'm dead, I won't know the difference. But many other people I care about will. So for them, I live as long as I can.