Some of this is because you are young and hormones are going crazy. It could also just be great physical atraction. Enjoy it while you can. I'm fortunate I can just think about my wife or she can just touch me just right and I'm ready to go and I'm 49 and still going strong.
You are just a healthy young male. A friend of mine once told me what it was like to be a young man. Everything gave him an erection.
Sounds like you are fairly young and may not be totally aware of what your body is doing. It is very common for this to happen expecially in youger men. While it can be embarrasing, it is perfectly normal. This will be less of a problem as you get older. As for when you are with your girlfriend, when you are just playing around, a erection is bound to happen.