part of it is that because you are mom, they just expect you to do for them. normal enough. time to put a little doubt in their heads.
sorry.....i FORGOT to wash your clothes. i was busy. like YOU forgot to feed the dog, and take out the trash.
no, i'm not giving you money/and/or a ride to go to the movies. you were a total asshole to me this morning and you hurt my feelings.
if they are old enough to be snots, they are old enough to know that their behavior has repercussions. even with mom.