[musouka]

I don't really understand why people assume that if she can't run off with Morita, then she will forever be missing out on love and be doomed to a terrible, unhappy life.

People change. Perhaps she'll find someone else to love who isn't either Shuu or Morita. The point is that Hagu made the choice that would allow her to continue what was most important to her at the time. Running away with Morita might have sounded romantic, but they were only in love in the most "broad" sense of the term. They didn't have much personal interaction, and I can't even begin to imagine the two of them having a "future" together, considering how irresponsible and inwardly directed both of them are.