I think whatever Aman said to his mother was because of her mental health, as before uttering those words they showed him having flashbacks of his talk with grandma and he contradicted his own statement in next scene by going ahead with the engagement, we can say they were cornered to do it but there wasn't even a little bit of hesitation on his part, on the other hand Roshni was very reluctant to do it and he noticed it. Roshni is very simple and innocent girl she doesn't know how to hide her emotions that's why she was failing so miserably at showing happiness despite wanting to be happy for others. And they have toned down Roshni's character, she was good even in her teasing mode, it was pleasant to watch her fangirling over her husband.