When a man is sexually stimulated, the brain sends signals that relax the smooth muscles around the arteries that supply blood to the spongy and cavernous bodies. The veins draining the bodies can't keep up, resulting in swelling. As the swelling reaches the limit of the penile skin, the penis becomes firm. The pressure of the spongy and cavernous bodies against the skin partially closes the veins, helping to maintain the erection.
Erection continues until the signals from the brain stop.
Nocturnal erections occur during all male dreams (regardless of what the dream is about), unless the man has physical problems (this is the easiest way to determine if impotence is physical or emotional in nature). "Morning Wood" is the result of being wakened during, or just after a dream; and it can be a very persistent erection. While a morning erection is not a sign of arousal, it's presents and the pleasurable sensations it can create may result in arousal.
MEN HAVE LIMITED CONTROL OVER THEIR ERECTIONS
During puberty the young man is often embarrassed by erection in public settings, but he gradually becomes able to suppress erections when the stimulation is mild. Likewise, it is impossible to "will" an erection, although sexual thoughts can cause erection. During prolonged foreplay a man's erection may go away; this is normal, and is not a sign of lessening interest.
