A couple reasons, some of which are personal.  One major one reason I'm comfortable talking about is I feel that I owe 3 people who took a chance hiring me.  They have been scrupulously fair in all their dealings with me, treated me well, and tolerated those parts of me that are not the perfect salaryman.  I promised them 2 ~ 3 years, and they've done nothing to make me consider breaking that.