What was the reason for Dean and Cindy's marriage to fall apart in Blue Valentine? Seems like Dean was ready to change his ways. Why didn't Cindy give him a chance?
In my opinion, Cindy's life is not going as she expected: she was studying medicine and then got pregnant; she tried abortion, but had to stop halfaway, so now she is a nurse.
At the beginning Dean's attitude made her fell safe, then she started to feel suffocated by his lack of initiative: he is always saying something like
You tell me what to do, and I do it!
this is irritating in every person, even more in a husband: he should cooperate not put all their relationship on her shoulder; in the long run this killed her respect for him and their relationship.
I would say he is kind of childish.
From the Rotten Tomato analysis:
A complex portrait of a contemporary American marriage, "Blue Valentine" tells the story of Dean and Cindy, a couple who have been together for several years but who are at an impasse in their relationship. While Cindy has blossomed into a woman with opportunities and options, David is still the same person he was when they met, and he is unable to accept either Cindy's growth or his lack of it.