The thing is the economy has rebounded and if that was the only thing I cared about, I'd be happy with Obama's performance thus far. But the problem is he never prosecuted any of the people who caused a global financial crisis, money in politics is bigger than ever, corporate tax loopholes are still very much a thing, income inequality hasn't gotten any better, and it doesn't seem like he's done anything to actually fix the root problems of our economy because he constantly bows down to the interests of the wealthy. Obviously nothing is going to get done now with the results of the recent elections but it's not like he never had a chance to accomplish these things.
Which is why it's hilarious when people call him a socialist or what have you, it couldn't be any further from the truth.
In the end maybe his initial goals were unrealistic, but a lot of people were hoping that he would address these issues and he just left a lot of young voters feeling cynical is my guess.