Of course,mothers is also the argument that it is impossible to do anything other than self gratify.
If you do something out of "moral duty" ... there is the argument that your small sacrifice makes you feel better about yourself that if you had done the other thing, the one you really wanted to do.
In fact, the personal gratification from doing the thing you felt you ought to do, instead of the thing you wanted to do, provides a personal utilitarian factor greater than if you'd gone off and had a good time.
You feel good about yourself.
Or, to put it another way ... you're a smug git.
Which makes you feel happy.
Whereas, if you go off and have fun, then you feel guilty.
Which makes you feel sad.
So you have a better, happier time doing the thing you "ought" to do, rather than the thing you "want" to do.