Sonakshi promised him that she wont leave him but she cannot do it alone, he has to stand by her. Dev promised he will do so.
Sonakshi from day 1 tried to fit in Dixit mad house, she just looked at the positive side of every situation.
Dev on other hand tried to make evry moment special for Sonakshi because he knew that she is not getting her due love and respect from his family members. But but but Dev babu is Dev babu, impulsive and short sighted.
Gradually because of some inevitable situations ( infertility and loan). Dev and Sonakshi both wanted their partner to understand them.
Sonakshi wanted Dev to stand for her and once take decision for them without depending on his mother. She wanted a long term solution to that whole mess.
Dev on other hand being impulsive just told Sonakshi to leave from his house at that moment and his short sightedness nature made him say that mother has left decision on me and i want to stay with u, not even once he thought how after all this deep mess, Dev, Sonakshi and Ishwari will live under one roof. Dev wanted that Sonakshi should understand and just stay with him .
So, here after 7 years both are blaming the other for failure of their relationship