Motive
Maybe he's doing it because he likes you. Maybe he feels that just because that's his star sign he can still be nice for other reaons, never question a guy doing nice things for you because it doesn't happen that often anymore.
A reason for not doing something could be lack of time, conflicting priorities, or it not aligning with your values or goals.
Purpose can refer to when someone has a reason for doing something. If something has a purpose, then there is a use for it, and a reason for it to be around, and in use.
arbitrary
Motive
one reason is because she is properly doing something she has no business doing
"Purpose", maybe?
purpose
Justified.
Spontaneous
Character Motives is what it is called.