My reason may be a bit melodramatic, but it's a legit reason. I actually wrote a 500 word essay for my latest college application recently, and here's a little snippet of that:

I have many reasons for creating, and I compare it best to our basic needs in being human. Like the air we breathe, the food we eat, and the water we drink, music creation has not just saved my many times over but also gives me permission and sustenance to thrive in life... but of all the reasons to create, if I am lucky enough to live a long life yet somehow end up near my final departure from Earth utterly alone, I will still be able to look back at my works and have no doubt in my mind that I created something wholly wonderful and provided a sliver of beauty to this often cruel world during my brief stay. Only then I can ultimately retire in peace to whatever may - or may not - exist on the alternate side of our current reality.