The first, and I'll admit-seemingly conceited, thing that comes to mind for me is, "I cook, because I can." That's true and not so arrogant as it comes off ... really.
I cook because I love to eat.
I cook to nourish my family in the best way possible. Letting someone else do it removes me from the process of knowing exactly what goes into our meals.
I cook to earn money. I develop recipes and take photos for a living 9and write about the process), and that end of it is ever fascinating and ever educational. I learn something new each day by doing.
I cook to remember. There is nothing so memory provoking as the aroma or flavor of a food loved as a child. Nothing. I make my grandmother's brownies or my great-grandmother's brown sugar cookies or my mother's fried chicken to recall all that was good and pure and wonderful about my childhood - even if it wasn't all good or pure or wonderful.
I cook to create new memories. Yes, the common thread here is that food is central to my feelings of well-being and health and I aim to continue building those feelings in my children and their children. I want them to have memories of love and home and nurturing all rolled into one loaf of bread or layer of cake. The more I cook, the more chances they have to watch and learn and begin to cook for themselves.
I cook to live. Eating is truly secondary for me to the creation process that takes place each time I make a dish. I tell people often that cooking is an art like no other with an instant gratification (or let-down, whichever the case may be) that you can't get by creating with any other medium. Paintings can take days to months to complete. Books are the same. Recordings, movies, sculptures - they all take much more time to create and to be appreciated. With food, the 'yay' or 'nay' are immediate - you know if your creation was loved or liked or hated before the creation is gone.
I cook to bring others happiness. There's nothing else to say about that.
So, I echo Michael Ruhlman - what are your reasons for cooking?