Obligatory describes something you do because you have to, not because you want to. When you buy a car, you have to fill out the obligatory forms. In many homes, saying please and thank you is obligatory.
Obligatory derives from the verb oblige, as in "after she was so nice, I felt obliged to take her a present." Some people feel burdened by lives of common obligation, giving presents, returning phone calls, eating birthday cake, attending religious services––none of which they feel any real desire to do.