The villain, of course, can set out to conquer the world with no problem, because arrogance is villainous. The hero who volunteers is not so lucky. And, of course, he still needs a motive. Revenge is popular for good reason. Or claiming the throne, which would explain all those secret legacies. A motive of pure heroism, indeed, raises the question of why any given evil is the one tackled.
Scaling down the matter is also a good technique, leading to less powerful motivations being needed, but it leads to its own dramatic problem, in that the smaller the stakes, the weaker the conflict.
Trapping the character is probably best. Either up front make clear that he needs this to escape some dire alternative, or have him tripping merrily into a situation he thinks small only to trigger large effects that force him to keep on trunking down the path he started on.