@SveNo You are right, but remember that she broke up with her last boyfriend because she wanted to get married and he didn´t. She loves the situation she's in right now and wants to protect it, so it makes sense she's afraid (even unconsciously) that the same would happen if she brought it up...
Finishing this is like saying goodbye to an old friend. So, so sad yet also nostalgic and wholesome. Flat is one of those manga that really fele like life. No goal, no hook, just people living inconsequential, yet fulfilling lives.