My signature gives a big reason...
But it's so much more than that. I want to live a long, healthy life and at the weight I'm at now, I just don't think I would live a life that is as enjoyable (or as long).
Also, I love being able to wear nice clothes when I'm in shape, right now I absolutely abhor clothes shopping, it's just depressing.
There's comfort issues too. We fly internationally a lot and those economy sets are NOT fun if you're carrying a few extra pounds!
I find at work people take me more seriously if I'm at a healthy weight.
Plus, I LOVE running. I definitely get the runner's high and it's something that I honestly look forward to. I agree with the PP who said that if you're not looking forward to it that it's not the right type of exercise for you. Now, it could be that you need to train a bit more because when you first start running you feel every stride! However, after a few weeks it gets much easier and much more enjoyable.
Exercise is easy for me. It's eating healthy that's not. Better put avoiding sweets. I'm trying to go sugar-free today and all I can think about is the half-eat bag of M&Ms in my desk at work. That's purely for the weight loss, because I don't find anything enjoyable about not eating chocolate!
ETA: I also agree about being a good example to my daughter. Since DH and I have been taking exercise much more seriously we've seen her imitating us and wanting to do sit-ups, push-ups, and run more. It's great to see that!