There are several things this could be.
All things considered, you could have one or several of many causes.
Yes you could still just have an above average (probably a good thing) hormone level and ability to have an erection on and off command.
Another could be that you're masterbating too much. I had a similar problem when I was 20 I thought that if I masturbated when I got an errection, it'd stop. I later found out that it was the opposite. If I masterbated it was worse, but sex didn't affect it. So, when I realized that masterbation was an issue with my having too many errections, it slowed down from 5-10 a day to about 2.
Another issue could be that your mind is in the gutter. A big help for me was to train myself to not fantasize about people I see everyday. This would make it worse. IE: A girl at work that I thought was smoking hot I would j/o to. When I saw her at work, bingo psychology shows that this is a branded pattern when you teach your body to do something when you have a reaction it will learn to do it without thinking.
And finally there is the unlikely, but possible chance of enlarged prostate! Do you have urination issues? Most older men with this issue have ED which is the opposite of what you have, but in some cases, an enlarged prostate could give you an errection when: You have to void (urinate), make a bowl movement (number 2), in some cases how you sit. And all of the things above that would lead towards an errection!
Hope this helps. The best thing you can do is ask your doctor, if your doctor doesn't know, get a referrel or make an appointment to see a Urologist. Don't be affraid to ask your physician these questions. Your paying for their mercedez so they are their for you. And there are always the HIPAA laws in the US if your worried that your doc will tell others about your member!