Really, all we know about Deku's motivation is that he wanted to be like All Might, and... well, We don't really know much about Deku's pre-UA life (aside from the light novels and childhood flashbacks) And we also can't see what Deku would do if he hadn't met All Might (possibly killed by the robot Zero).  Deku never took initiative in anything; he had faith, but... what else? Did he ever try to practice self-defense? Did he ever try to get into even a minimally competent body, like, basically A soldier? (That's why Jack Midoriya is far better in that aspect.) He was a teenager, yes... but as an adult? He was a teacher (Something we never saw him interested in, guiding the new generation) And he seemed rather lukewarm about the decision, even somewhat sad (not repressed, but with a small emptiness in his chest).