Pub Fight

Fight in the pub is the main reason which is making Ratan behave in this weird manner...Ratan being little immature and due to lack of experience is still stuck with Diya's taunt of him being useless and that is the reason he has become little vindictive towards Diya where as Diya being a mature person has somewhat forgotten that fight or may be knows that she got carried away in that fight and has hurt Ratan so she is trying to mend this relationship.

Deep down Ratan still consider Diya to be his best friend but its Diya's taunt of him being useless which he can't overcome and that is why he is behaving so rudly with Diya.

After Ratan came back to India Diya was one of his biggest support system and now he is at logger heads with her ...so uska thoda pagal hona toh banta hai🤣

Actually in real life when you think that a person believes in your capability but other day you realize that she/he doesn't than you would freak out.

both have some weakness and sore areas. For Ratan being called useless is his. For Diya, being called boring pricks her as does the point on her age. We have seen it again and again in the show.

Yes actually this is probably the exact reason why things are going so so wrong with DiRa as both had a massive fight with each other and made some extremely hurtful comments about each other. Ratan always thought of Diya as the one person who believed in him the most and always wished for his happiness but during the fight she insulted him in front of everyone and said stuff that was derogatory to his self esteem. That can very well be the reason that he is so pissed with Diya and that's why he's so rude to him as he probably thinks that Diya thinks of him as a good-for-nothing irresponsible chap... (not that this justifies his act of dragging Diya downstairs in her bathrobe for the portrait... that was just plain rude... it was just too bad...)