why do we change

i don't get, that life changes for people when they get a boyfriend or girlfriend? how everything you believed isn't important anymore? the saying is true, we become who we hang out with. my daughter has gone from an active girl, riding her bikes everywhere,going to the gym 3 or 4 times a week, and really eating for nutrition for 90% of the time to a guy glued to her side, getting taxied around everywhere by him, not going to the gym, laying in bed watching tv with him eating chocolate and ice cream. and you can no longer get a conversation in  without his ears listening too. :/ and she says nothing has changed. ? ! :(