Erectile Dysfunction -- clinically speaking -- is usually caused by one of, or a combination of, three different possible things.
1) Neuromuscular -- damage to nerves and/or muscles responsible for controlling the functions of the penis.
2) Circulatory problems -- where the cardiovascular system is having problems with the blood flow or blood pressure required to maintain an erection
3) Psychological -- when psychological factors (which may not be conscious at all) interfere with erection, maintaining an erection, or arousal.
Smoking, over the long term, can predispose someone to cardiovascular problems that can cause ED in some cases. But it's simply not as simple as "smoke cigarettes, get erectile dysfunction."
My guess is that what's going on here is probably mostly psychological in origin... with younger men, that's often the case. But it's really nothing to be too fretful about. Almost all men experience erectile dysfunction at least SOMETIME during their lives. It's more common in men over 40, but it is certainly not UNCOMMON in men of any age.
WOmen as well as men can experience psychological or psychosomatic problems with arousal or with maintaining arousal. Factors that can contribute to psychological (or psychosomatic) ED or other arousal problems include:
lack of sleep
worry about sex
overwhelming worries about other life issues
fearfulness that sex will cause problems or bring on repercussions (pregnancy or STD fears are often common, as well as fears that sex is "bad" or "wrong", also fear of getting caught is a biggie)
Basically, if the situation is not optimal in terms of physical preparation (sleep and stress), or if there are things going on that are distracting someone, they are going to be much more likely to have problems getting aroused or staying aroused.
TALKING to your boyfriend about what's going on can help. Often, if men have erectile issues, they feel really horrible about it or may feel like they're not "real men" or something like that. These feelings can make things even worse! So do talk about it.
Two other things to remember:
1) Erectile difficulties are most likely not about YOU any more than it would necessarily be about your partner if you weren't in the mood, were distracted, were upset or angry or depressed, or hadn't gotten enough sleep the week before due to life stress.
2) It is not necessary for a man to have an erection in order for him to feel physical and sexual pleasure. The entire body is a sensual thing. That goes for women's bodies as well. A hard penis is not a prerequisite for two people to share sexuality.
Start a Revolution -- Stop Hating Your Body!