The hormonal system influences desire and erections by such hormones as testosterone, leutinizing hormone, and prolactin. Very low levels of testosterone can disrupt sexual desire and sexual functioning. Possible causes include a systemic hormonal problem, fatigue and stress, alcohol and drug abuse, or a pituitary tumor.
Clear your mind of preconceived notions and look at your penis as if you have not seen it before. The erotic stimulus for your erection may come from your brain via sexual thoughts and fantasies or from direct physical touch. The first kind of erection, called psychogenic, spontaneous, or automatic, is common in younger men. As you become older - about 35 or 40 - erection usually requires direct stimulation to your penis. This is not an indication of a problem but is a natural change that occurs with aging and can enhance sensation and pleasure.