Fitzgerald and Meredith in their book Structuring Your Novel write that nine out of ten published first novels are about personal experience. I think this is because the best writing is often about subjects with which the writer is most familiar and about which he or she is most passionate. Few subjects better meet those criteria than one's own life. I think also that writing about personal experience is cathartic. It helps the writer put the past into clearer perspective. It helps the writer get to know himself better. As others have said above, its therapeutic.
I write because I have to, because I love it. The writing I enjoy the most, being a narcissist, is work about myself, or, at least, work inspired by my life and feelings. I find it more difficult to write about a subject that has nothing to do with me unless I find some connection, however tenuous, to my life, emotions, or feelings.
I hope this didn't sound pretentious! :)