Saving people was his purpose. With the concept of heroism being so tightly threaded into who he was as a person, it came as no surprise that he would succeed as a pro hero. That said, no one could say they expected him to be brought on as a teacher at his alma mater. That couldn't be a good idea, right?
Just because all those Mystery Squares popped up and started throwing random numbers and rules and stuff in Luffy's face didn't mean that he was gonna listen to them. (Gamer parody. Not a crackfic. I'm only half sorry for writing this.)
Karasuba never had any intention of finding her "Destined One"; to spill blood, any Ashikabi would do. With that said, she was never supposed to meet a genocidal maniac as sick and twisted as Emiya Shirou. Maybe romance wasn't dead after all.