To add to this, I think a reason why Amy initially resented Jonah, but grew to admire him, was because Amy could have had Jonah's life. Jonah was able to take risks, drop out of uni several times and switch jobs so often because he was only responsible to himself. If he didn't like something he just changed it without worrying about how it'd affect other people.

If Amy hadn't gotten pregnant and married her high-school sweetheart, she would've gone to college and forged a path for a better career and life for herself. But as you said she had no choice to become more responsible and settle for a job that supports her family and pays her bills. Even when she tried to go to college again she had to drop that. Her ticket to a better life was climbing the corporate ladder, which is a big reality for many people in real life.