There's really no rhyme or reason in our generation as to how things are supposed to work. I don't believe in the words supposed to. I believe in everybody having their own path and their own learning experience. To each his own.
When you're with somebody for a long time or have a child with someone, you start to expect them to be this person you had in mind. Then if they don't fill the expectation, resentment starts to come up.
Relationships teach you what you do and don't want, what you can and can't put up with, what you do and don't deserve.
I learned a long time ago that I can't live my life for everybody else. We learn and we grow from our mistakes, and we get older and more mature.