I agree. It is quite common and especially at your age it is a sign of good health. And it is just natural that it happens more whene there is around the significant other. Sometimes we can get it more under control when we have regular sex and when the frequency of sexual activities corresponds with our needs, or at least regular masturbation can sometimes help. The erections and sexual arousal are a natural sign that our body is ready to have sex.
It's not at all uncommon. And, I really see nothing wrong with it. The man who I'm involved with is 50 years old and he's the exact same way... easily aroused. My guy is a letter carrier and often talks to me on his cell phone while he's delivering mail. However, the minute I mention something about sex... he starts to grow! So, you're right... erections can occur in the most inopportune time and lead to embarrassing moments. You just have to try to curtail your thoughts... particularly, when you're in public.