In the beginning of A Separation, Nader didn't really want to get divorced and was only trying to oppose his wife's decision seemingly wanting to convince her his own way. But, at the middle of the film, he started to change his mind and he got really serious about going through with this decision. What made him, a character who doesn't seem to make any decisions without enough reasons, change his mind?
Initially, Simin (the wife) wasn't completely serious about her threat of divorce. She was merely hoping to force Nader's (the husband's) hand and get him to agree to leave Iran with her and Termeh (their daughter).
During the movie, we are told several times that Termeh doesn't want to leave Nader for Simin. At around the 1:43 mark, we learn that this is in fact Termeh's tactic for getting her parents to stay together -- Termeh knows that if she doesn't go with her mother, then her mother won't leave:
Nader: If she was suffering she wouldn't have stayed.
Simin: Why do you think she stayed? Did she choose you? To keep us from separating. She knows I won't go anywhere without her.
During the movie, the family undergoes a series of unfortunate events that put further strain on their relationships.
Between the final blood money discussion and the final divorce scene, we are not told what happens or how much time elapses. However, one can easily imagine that following the unhappy events of this movie:
Termeh had witnessed her father pushing a pregnant woman and then lie about it. She was then also made to lie about it in order to protect her father. (As was clearly depicted, having to tell this lie was deeply traumatic for her.) Her father then subsequently also escalated the situation by, for example, initially refusing to pay the 15M blood money. This caused further strain on the family. Termeh was thus probably less enamored of her father at the end of the movie than at the start of it.
Simin decides that she'll go ahead and leave Iran with or without Termeh. (She probably also guesses -- and probably correctly -- that Termeh will come with her anyway.)
Nader realizes all this and that there is no point trying to stop Simin. He is probably also a little chastised and weakened after the events depicted. He thus agrees to the divorce.
You know that's what love is. Well first of all you need to understand how life and relationships work around in Iran. Its not America! So here's the thing:
Usually once you're married for a long time (for more than 5-10 years) and then want a divorce, you probably start to think about the future of your kids. Not only this but in well-cultured families the people fear how society will react to their decision.Also if your financially struggling, it definitely adds to the pressure.The movie starts off in that fashion where Nader has not yet given a thought to the future, hence he tries to review the decision made which will affect him, his wife and his children.
Men do care you know ;-)