I'm sure people will just love to complain about the sudden ending, but in my own humble opinion, this ending is just fine. It fits the manga, the story has been pretty generic fluffy romance so far anyway. As a writer, it's a much better decision to just end the story when and how you want instead of dragging it out while you run out of ideas. Just be happy the series didn't introduce unnecessary characters and love triangles just to draw out the runtime. When a good series ends people will always scream for more thinking that that's what they want.
Tl;dr: Ending's fine, deal with it!