Another reason why the girls are wary of the MC is thanks to his kingdom's bad reputation (Even the MC himself recognized that) and thanks to him being the third prince, the probability of him sharing the same ideology is real and he could be just faking a nice guy persona to make them lower their guards.
Anyway about what the guy can gain:
1 - obviously since he's a third prince he'll never go to the throne, unless his two older brothers are killed or a civil war happens and he emerge victorious with some nobles backing,
2 - if he returns he either will become his older brother vassal until he dies or will be assassinated in a war of sucesssion
3 - it seems his own family, aside of his deceased mother, don't pay much attention to him and my guess is that his older brothers sent him to get killed by the Dragon and didn't expect him to emerge victorious.
So instead of he instead could rescue this kingdom become the Hero and then the King, on top of having 7 beautiful and high standing wives if this story ends in harem.
And since this particular Kingdom was put to sleep, something tells me that those girls are more valuable, maybe due to some unknown potential power or stuff like that.
