When one perceives something as an obligation, the yetzer hara comes out and uses its guile to try to get the person to refrain from doing it. Something that one perceives as voluntary (read that privilege) there is no need for the yetzer hara to work so hard.
Hashem gave us tzvuyim – commandments – obligations – in order to give us schar. If we just want to do things that we like/want to do, then the schar is less, really for whom are you doing it?