Not being turned on is a main reason for not getting an erection. Also, age, to many drugs, medication, alcohol, job stress, physical exertion and the list goes on and on. The male reproductive system is really more complicated than once thought and many factors can cause a man not to get an erection. You see so many ads in magazines and on the TV these days to correct ED and that is only the surface of the problem. Just be happy your bf performs well with you and enjoy it.