When my wife visited USA after years of living in Poland, question about reason of the trip was not even asked, or it went so swiftly I don't even remember*. She is a citizen, so the reason of her trip to her country was no problem at all.
Question that was asked, and topic that was explored by the officer was "What kept you away from your country so long?". You must be prepared to answer this question truthfully.
My wife had two reasons, and you seem to share one of them. When she told the officer that her reason was education she could afford without getting student debt he seemed to be quite satisfied. You needed to finish your high school, you probably got someone to support you there and no one in the USA, et cetera.
So, what you have to do is to think about honest and likely reasons you stayed away from your USA homeland this long, and why such reasons ceased to apply. It has to be truth, and it should be given in a way that rings no alarm bells on the border. They can't deny you entry anyway, but it will make it easier for you, for the officer, and for the people in line behind you.
* She was on the wheelchair then so I was allowed with her as an assistant for the disabled person.