I can tell you that when I was fat, I hated exercise.  I was always out of breath and everything hurt.  And since I was eating like crap, I was never improving, so I didn't get to see the benefits.  It wasn't until I cleaned up my diet and could start seeing  myself lose weight,  get stronger, and run faster, that everything clicked. now I'm at the point where I get grumpy whenever I get cheated out of a workout day.