The answer is a little complicated because age plays a factor in the equation as well. A man in his early twenties, thirties and into his forties, who has never really experienced problems with getting an erection, may find this to slowly be a problem as he ages. Once this happens as a man ages, he may begin to worry about it, creating preformance anxiety.
Most problems with maintaining erections are psychological, but can be based in a medical or physical place to start with. As men age with the stress of the economy, kids, possible divorce, health issues,dating again, it can have a direct affect on his libido and performance. He may be too exhausted, fatigued or anxious to get fully aroused or have his penis function on command. This is all normal and for the most part it will pass, but what becomes the underling problem for some men is the anxiety surrounding it, which then leads into the performance problem. This then leads the man to go to the doctor for a prescription for Viagra.