Only through a journey of self discovery, and some random action he finds himself thrown into, does Stephen Strange eventually decide to become a sorcerer dedicated to protecting the Earth.
With Doctor Strange, our protagonist is driven purely out of personal pride, longing to just become a “normal” person again.
In the Marvel Cinematic Universe, we’ve seen heroes that have either fought to become the super-powered beings, or become heroes out of sheer necessity.
By time he faces Dormammu, he’s willing to sacrifice his own life to annoy a being beyond space and time, just so we can live freely.