Practicing abstinence is great for a teen's self image. Let's say that you had a tendency to eat out all the time or be a couch potato. Well abstaining from eating out and starting to exercise can make you look and feel great (along with saving money!). Now the teen has to make the decision for themselves or forcing them to not do the things they normally like to do will make them think that their parent is a trying to be mean, or that their parents are just trying to keep them from the things they love. Practicing abstinence also helps you control your impulses. So maybe you were going to make an impulse buy, but you thought about wether you really needed it or not. Well, you didn't so you just saved money! Congrats!
Hope this helps!
