DR Halo kind of implied that ED is just a blood flow/health issue. There are a hundred other things that affect a man's ability to get and maintain an erection. First, and possibly the most important aspect he has to be aroused mentally. Just because he would like to have sex if his mind doesn't form some sort of erotic thought it might not happen.
Medications can kill the ability to get an erection and orgasm.
Body image both male and female
Stress at work and in the home.
Depression
The anticipation of failure to perform
Criticism
Mental, mental, mental, all it takes is one little thing to trip us up.