Many men are under the belief that low prolactin is a good thing and will help them in the bedroom. This comes from several studies on different subpopulations of male participtants using cabergoline, a prolactin-lowering medication. The results were impressive in many cases:
1. Men with High Prolactin. Almost all the men in the study had their testosterone restored - high prolactin lowers T - and cabergoline not only restored their testosterone but usually their sexual function as well. 
2. Men with Psychogenic Erectile Dysfunction. Cabergoline helped in many key areas that us men are interested in. For example, it improved a) testosterone, b) erectile function, c) libido and d) orgasmic function. 
3. Decrease of Refractory Period. According to one animal study, lowering prolactin can decrease the refractory period, which is the time necessary for recuperation between orgasms.  (For more information, see my link on the Male Refractory Period.)
Now just in case you reading this and about to run out and order some from the nearest offshore pharmacy, I should point out that cabergoline is full of side effects. It would probably be a much more popular medication if this were not the case. Furthermore, it is one thing to have normal or slightly high prolactin levels and take cabergoline and still another to have low prolactin levels to start with. One cannot simply assume that low prolactin is always a good thing and any man with low prolactin levels would be a high testosterone, high libido bedroom machine. Unfortunately, this is not the case for reasons I will discuss below - quite the opposite actually. In fact, low prolactin is very often the sign of underlying medical conditions.
NOTE: Elevated prolactin can lower testosterone and, if high enough, lead to a type of secondary hypogonadism. For more information, see my page on Testosterone and Prolactin for more information.
The reason that low prolactin can so negatively affect your sex life was shown in a 2009 study in the Journal of Sexual Medicine that divided over 2,500 men with sexual dysfunction into quartiles and found that the low prolactin men were: 
Let's start with the relationship of low prolactin with Metabolic Syndrome. I call Erectile Dysfunction Syndrome, because it is SO hard on your sex life. Metabolic Syndrome, a.k.a MetS or prediabetes, is a plague in modern societies that includes a common suite of symptoms such as insulin resistance, high triglycerides and hypertension. So why would low prolactin levels be related to prediabetes?
Researchers are just getting their arms around the subject, but it appears that prolactin improves insulin release and "supports the growth of pancreatic islets", both of which will decrease risk for diabetes and prediabetes.  This probably also explains to the increased risk for arteriosclerosis and erectile dysfunction in the above study. Both diabetes and prediabetes are risk factors for heart disease and impotence.
NOTE: High prolactin has been shown in multiple studies to lead to heart disease as well. 
Why would low prolactin levels lead to premature ejaculation? As mentioned above, prolactin levels are very tied to one's post-intercourse recovery time: the higher the prolactin, the longer the recovery time. (Nitric oxide levels seem to also play a role.) And, apparently, low prolactin levels can lead to a very short recovery time and premature ejaculation as well. This condition can be very hard on relationships and increase anxiety and depression.
So what was the lowest quartile? It was prolactin levels below 113 mU/L or 5 ng/ml. For this reason, if you having any kind of erectile dysfunction or loss of libido and, if you also have low prolactin levels, this may provide you a clue as to how to troubleshoot the underlying issue.
NOTE: One animal study indicates that a zinc deficiency can lead to low prolactin levels, so you may want to get that checked.  Some men do a taste test for zinc testing. Be cautious, though, with raising levels too high as zinc may be neurotoxic and cause many other problems if one goes too high. See my page The Potential Dangers of Zinc for more information.
Low prolactin can in some cases be a sign of underlying medical disorders, which is why it's always important to work with a physician when you have low prolactin. Here are a few examples:
1) Hemachromatosis. This very common genetic disorder - about 1 out of 200 in men of European ancestry - can lead to hypogonadism as I document in my link on Causes of Low Testosterone. It turns out that it can also lead to hypoprolactinemia, or clinically low prolactin. As the iron builds up, hypothalmic function goes down and, of course, the hypothalamus controls many pituitary pathways. By the way, one obvious solution would seem to be just donate blood to reduce iron stores. Unfortunately, this may not work according to one study, which noted that "no significant improvement was noted, however, in growth hormone or prolactin secretion after phlebotomy. Of the 11 patients, 7 had secondary hypogonadism, and phlebotomy did not improve the serum testosterone, follicle-stimulating hormone, luteinizing hormone, or responses to LHRH in any case." 
2) Pituitary Disorders and Head Trauma. Of course, head injuries, tumors of the pituitary, tumors near the pituitary and other disorders can affect prolactin output. Discusss with an endocrinologist if you have any concerns.
3) Medications. Many medications can lower (or raise) prolactin levels and so you should consult with your physician if you are on medications and your prolactin is low. Check out this study for a sample of all the medications that can affect prolactin. Remember that anything that affects dopamine is likely to affect prolactin as well.
1) The Journal of Clinical Endocrinology & Metabolism February 1, 2004 vol. 89 no. 2 621-625, "Six Months of Treatment with Cabergoline Restores Sexual Potency in Hyperprolactinemic Males: An Open Longitudinal Study Monitoring Nocturnal Penile Tumescence"
2) International Journal of Impotence Research, 2007, 19:104 107, "Cabergoline treatment in men with psychogenic erectile dysfunction: a randomized, double-blind, placebo-controlled study"
3) Behav Brain Res, 1984 Jun, 12(3):267-73, "Brain monoaminergic control of male reproductive behavior. II. Dopamine and the post-ejaculatory refractory period"
4) J Sex Med, 2009 May, 6(5):1457-66, "Hypoprolactinemia: a new clinical syndrome in patients with sexual dysfunction"
5) Trends in Endocrinology & Metabolism, Apr 2006, 17(3):110-116, "Focus on prolactin as a metabolic hormone"
6) Hypertension, 2009, 54:98-105, "Prolactin and Preclinical Atherosclerosis in Menopausal Women With Cardiovascular Risk Factors"
7) Mayo Clinic Proceedings, Jun 1987, 62(6):473 479, "Influence of Phlebotomy Treatment on Abnormal Hypothalamic-Pituitary Function in Genetic Hemochromatosis"
8) Nahrung, 1989, 33(10):935-40, "Role of zinc in regulating the testicular function. Part 1. Effect of dietary zinc deficiency on serum levels of gonadotropins, prolactin and testosterone in male albino rats"