"We gain the strength of the temptation we resist."
Ralph Waldo Emerson, 19th century American essayist/lecturer/poet.
(You think this pup would agree?)
Temptation comes in many forms and is generally attributable to things not good for us.
(Did you ever hear of anyone resisting the temptation to eat broccoli?)
I'm not certain I've ever gained strength from resisting something I knew I really shouldn't have, but I do know I've later felt better when I did, with the reverse true when I didn't.