Because it was Bijoy's daughter who invited Dev to come and stay with them .If he has any issues then he needs to talk to his daughter first not show his frustration on Dev .Like you said he is not the SIL anymore so they have no right on him nor does Dev have on anyone except Soha .Another reason is no matter what differences Bijoy has with Dev fact is he is Soha's father and like it or not he needs to respect that .If he is having a problem then talk to his daughter and ask her to make different arrangements
Apart from age has Bijoy done anything to gain respect in Dev's eyes or even among his family members ? Would you have any high regards for someone who is acting childish by not serving food for guest,who doesn't spare a innocent child and lashes out at him , try to snatch your daughter from you on the pretext of saving her ,who is ready to create a scene anywhere and everywhere , enjoys food but still makes snide remark about the food .
No one is expecting him to love Dev but having some courtesy for ur guest is needed .If he cannot forget accidental slap then it's his choice but he cannot use that to insult Dev and make everyone around him miserable with his actions especially Soha
Ok. So we are going by " athithi devo bhava"... here he is athidi and also dev .. so i can understand...
Since we are talking about expectations. Sona and dev were discussing multiple options and dev clearly said he doesnt agree to the 7 day deal and discussions ended for that day. Then all of a sudden he decides to take the deal and drops in with out informing- knowing bijoy doesnt like him .. he intentionally tried to aggravate him by bringing the bp tablets (i am sure he expected bijoys reaction and wanted to provoke him further - unless dev is a bp patient and he carries his medicines in his pocket). So after this if bijoy played a similar prank on dev it is not acceptable!!
Now coming to current situation... bijoy caught dev with jewelry bag and dev accepted that he did it and doesnt know why he did it - no explanation. Should. Bijoy just forget everything and be normal with him ?
Coming to snatching daughter... i remember dev being nasty to bijoy over phone even on the day he locked up sona in office. He broke sonas name plate that carried bijoys name. Until then, bijoy was not nasty with dev .. i dont see any mention of the slap from anyone in bose family in S2. It is not the slap that worries bijoy .. it is that dev will hurt sona and soha once again. So snatching the daughter... if dev can respect sohas wishes and agree to all that soha is asking for.. why cant bijoy respect sohas requests.. he did not take soha away forcibly... i think giving into a childs wishes like this is wrong by both bijoy and dev - that's a separate rant.
So i still dont see any solid reason why bijoy should be nice to dev and dev should be nasty to bijoy..