If truth be known I stopped asking myself that question sometime back. I don't think it was because I rationalized an answer. Maybe I even stopped asking the question or perhaps it was that I actually stopped caring. I'm not sure.
It is quite possible that I felt the answer and didn't know it. Wasn't aware that I felt it and at the same time some internal satisfaction was met without realizing what or how the issue had been satisfactorily resolved.
Anais Nin once said, "If you do not breathe through writing, if you do not cry out in writing, or sing in writing, then don't write, because our culture has no use for it." I think in writing I have perhaps proven, to myself at least, that I do all the aforesaid verbs. I think that I have stopped questioning "why write" because it has become for me the very photosynthesis that allows me life.