Apparently in the light novel the explanation is that whatever he thinks is super obvious to anyone looking because of how it shows on his face (at least that's what other commenters have said). So those are narrative boxes, but they might as well be dialogue boxes.
Psst... hey, MC... You think she's hot. You've openly told her she's hot. She thinks you're hot and openly tells you you're hot. From what I can tell, both of you are single. Stop making this difficult.
After learning just who the author of this manga is... you're honestly better off dropping it. You'll lose IQ points if you keep going. Good luck in whatever you decide to pick up next!
Knowing the author's other title, that's going to get harder and harder to do... eventually you'll just have to pretend the whole series never existed.
Honestly I think the only way the request almost makes sense is if this is the reason. If Grandpa's a bad guy, he'd want the marriage for the prestige it brings his family. If he's a good guy, he'd want it because it's good. If it's a test of character, or a "you need to improve yourself first...