A lot of younger guys are not going to believe that some men can't orgasm. Nor would they believe that some men can get a decent erection but can't orgasm. I have read stories of men actually faking an orgasm due to this issue. Yeah, it's not just women!
The technical term for the inability to reach the Big O is anorgasmia, something that falls under the broad umbrella of orgasmic dysfunction. And,again, you younger guys are going to be shocked to know that the prevalence of this has been estimated to be between 8% and 14% of adult male population.  If you consider that an even higher percentage of men probably have delayed orgasms as well, this is a real issue and a common issue - one that almost rivals premature ejaculation. And it can be just as hard on relationships and self-esteem as premature ejaculation. Look at what one of our long time posters wrote and keep in mind that he is a young guy in his early 30's:
"One thing I've noticed since 09 is I dont orgasm fast....I dont go soft, I just keep going, till I get tired or usually bored. Ive been told I made the ex-gf feel ugly and it hurt her self-esteem(she was/is 8 yrs younger, so 22/23, in the prime of her physical appeal. I thought this was due to me being overweight..." 
Of course, a somewhat delayed orgasm and the accompanying control that comes with it can actually be a good thing for your relationships. The extreme, though, can really cause a lot of stress as this shows.
So what can make it to where you can't orgasm? Unfortunately, many things can create this kind of sexual dysfunction and this helps explain the high percentage of men that struggle with it. Below are some of the most Common Reasons for Anorgasmia. I am going to avoid some of the more obvious causes, such as prostate surgery and excessive alcohol consumption.
1. Medications. Many drugs can interfere with orgasm. The big culprits are antidepressants - especially SSRI's and tricyclics - alpha blockers, Tagamet, diuretics and opiates/painkillers. But there are many other less common ones, such as psychotropics including mood stabilizers, anti-anxiety, and schizonphrenia pharmaceuticals for examples. Discuss with your physician and/or pharmacist if applicable.
2. A Few Supplements. Although not as common, it is possible for certain supplements to induce delayed ejaculation or anorgasmia. Remember that, in general, increasing serotonin will increase the time to ejaculation, so any supplement, such as St. John's Wort, that addresses serotonin could do that. (And St. John's Wort will occasionally lead to this issue.) 
3. Elevated Prolactin. Elevated prolactin levels are quite common and can lead to delayed ejaculation and anorgasmia.  Prolactin function also regulates male recuperation time between orgasms, something I discuss in my link on The Male Refractory Period. Cabergoline, a prolactin-lowering medication is often recommended as an off-label method of improving anorgasmia. One recent study found that one third of men with anorgasmia improved with .5 mg of cabergoline twice a week and another third completely recovered.  Again, cabergoline has a fair amount of side effects, but this seems to show that prolactin is ground zero for many men's problems.
4. Neuropathy and Diabetes. Anything that affects the CNS (Central Nervous System) or induces neuropathy can make it to where a man simply cannot orgasm. Diabetes is notorious for causing this and diabetes often leads to neuropathy, so it is no wonder. For more information, see my page on Neuropathy and Erectile Dysfunction and Natural Neuropathy Remedies. Also, if you have retrograde ejaculation, i.e. almost no Semen Volume, then that is a possible sign you may have some nerve-relate injury going on. Discuss with your urologist.
5. Hypogonadism and Low Testosterone. Many commentators have noted that low testosterone can in some cases cause anorgasmia. And one study on testosterone pellets found that one of the participants reversed his anorgasmia with this kind of HRT. 
6. Nitric Oxide. Anecdotally, some men on The Peak Testosterone Forum have reported that improving nitric oxide can improve time to orgasm and other parameters - pardon the formality there - surrounding orgasm. For example, one of our senior members and an occasional guest author, known as Inigo, reported the following with an NO-boosting regimen in his Review of Prelox.
"But by the fourth week there was certainly an improvement in firmness and duration of erections...but little increase in speed of erection. There was also an increase in the pleasure of orgasms. (Prelox is marketed mainly as a pleasure enhancer )." 
Further evidence comes from the fact that a number of studies have shown that women can use PDE5 Inhibitors to overcome the anorgasmia associated with SSRI's and antidepressants. One does have to be careful though: one of the side effects of the PDE5 Inhibitors is anorgasmia.
7. Psychogenic (Psychological) Reasons. This is difficult to estimate, but a significant percentage of men are reported to be unable to achieve an orgasm due to anxiety and stress-related issues.
8. Dopamine. Anorgasmia probably has a dopamine-related component to it. Examples of this: Parkinson's patients sometimes cannot achieve an orgasm. And, of course, testosterone increases dopamine in the brain as well.
SOLUTIONS: Cabergoline is a medication rife with side effects. As mentioned above, it is reportedly very effective in its off label use of helping with things like the inability to achieve an orgasm and decreasing one's refractory period. (See #3 above.) Considering that cabergoline works so well, macuna pruriens may help as well (since it lowers prolactin a little). Of course, it would always be prudent to get a prolactin read before even considering such a therapy and the reason is that many men who have insulin / Metabolic Syndrome issues have low prolactin levels. You can read about this in my page on The Causes of Low Prolactin. And always discuss with a physcian of course.
Other solutions require a bit more experimentation and include changing/dropping medications (under your physician's guidance of course), testosterone therapies and NO-boosting supplements. See my links on Testosterone Therapies and Erectile Supplements for more information.
NOTE: Is your wife or girflfriend having trouble reaching orgasm? I have some basic information here: How to Help an Anorgasmic Woman. (Some women may require a "jumpstart" of their system.)
2) Prog Urol, 2008 Feb;18(1 Suppl FMC):F8-10, "Diagnosis of male anorgasmia"
3) Current Opinion in Urology, Nov 2011, 21(6):527 534, "Prolactin in men's health and disease"
4) Evid Based Mental Health, 2002, 5:111
5) Andrology and Gynecology: Current Research, "Recent Advances in Hypogonadism Urologic Clinics of North Alabama P.C., USA", by Amit Chakrabarty
6) Harvard Men's Health Watch, "A new option for orgasm problems in men", POSTED MAY 29, 2012, by Daniel Pendick