Think you r a woman and.
Your mother slaps you .. What will you do?
And your husband/partner slaps you.. What will u do?
You would definitely leave your husband stating your self respect but you won't leave and break ties with mother..
Above example is a bit of extreme but gist of my point is that every relationship has different expectations. Finally it's all about expectations and once your expectations break you tend to fall out of love and distant yourself from that person. For parent that expectation is different you may be ok with getting slap once twice but even in that case of its frequent you will leave but in case of partner you will leave even after once.. Dev had expectation from sona that she won't leave him watever happens and according to him he did not make prenup and did not send eviction notice. That was maybe unrealistic expectation Dev had from sona. He thought despite of his fault she won't leave. He took her for granted. That's not the case with ishwari. He thought his mother as God so he never had that level of expectations from ishwari instead he expected from himself to keep ishwari happy alwyas. So it's easy for him to overlook ishwari S mistakes..
As sona said relationships dnt compete with each other . They complete each other. Perhaps Dev feels hurt that sona left him because he wouldn't leave anyone even if they make mistakes. I am not sure though about it.