Word for doing something because you want to

Question

I forgot the name of an adjective which means that you do something because you want to. It has a complete anytonym which sounds similar (I think they just have different prefixes) but means you do it because of outside factors. Any help is appreciated :)

Answer 1

The word you are looking for is intrinsic, and its antonym is extrinsic:

Intrinsic motivation involves doing something because it's personally rewarding to you. Extrinsic motivation involves doing something because you want to earn a reward or avoid punishment.
_{Intrinsic and Extrinsic Motivation: What's the Difference?}

Answer 2

voluntary

done, made, brought about, undertaken, etc., of one's own accord or by free choice

His joining the army was voluntary, not because he was drafted.

The opposite is "involuntary."