Allow me to have a second go at this question. I had the insight that in writing music we create our own truth, so that we no longer are at the mercy of society "imposing" on us (or filling the gap in our theory of truth with) its own version of truth. So, I compose in order to create my own truth, my own inner truth (as against society's superficial truth), something I can do best through composition. I believe that at a fundamental level that is why we all compose - to create our own truth.
And here's my contribution of a funny reply:
"Because I don't want to "decompose"!
Yet it has a grain of truth in it because composition is great stimulation for our minds, and hence a composer's ceasing to compose may well spell the the beginning of the process of "decomposition" of their mind.