Top two reasons for me:
1) My health--Until this year I didn't have any of the typical smoker's problems (and I've been smoking 2 packs a day for 20 years), but recently I've become out of breath more easily and I had brochitis twice this winter when I usually don't get sick at all.
2) My father died of esophageal cancer, which was most likely caused by his smoking. And cancer runs strongly in my mom's side of the family.
I had tried all the other means with no success, but happened upon the e-cig and now I'm mostly quit analogs. Down to 3 or so a day, which is pretty good considering my former habit, and hope to be down to none by the end of the week.