Several things:
How I look, feel and function when I workout and eat well, vs when I don't. I also think back to a time when I let my exercising go for a while, and when I went to a conference and felt "dumpy, lumpy and doughy" and just unattractive. I don't want to get to that place again.
Habit: exercising and eating well has become a lifestyle and pretty much an automatic response.
Taste: I prefer the taste of healthy foods to the taste of unhealthy foods.
Age: As I grow older, I realise that bad habits have more of a negative effect on me than they would have in my 20's.
Vanity: I like looking better than many folks my age. Looking younger.
Negative role models: when I see the shape our former secretary is in (now diabetic, overweight, ailing, various aches and pains), I definitely want to avoid being like that. I pretty much can avoid what she does (eating high-fat processed foods) and do what she doesn't (exercise) to achieve an opposite effect!