It could be one of many reasons, or a combination of thereof. For instance, many times people say you're not smoking it right. This could be true, but it could also be your biochemistry coming into play. I've smoked weed before, and I think I've only been high once or twice, and never enjoyed it. It's just not my thing. Also, I don't feel different on prescription narcotics and painkillers. But, I don't normally abuse those drugs although I seem to have access to prescription narcotics often times, like a codine cough syrup!
However, size, height, metabolism, body fat percentage, etc also should be taken into account. But, the chemicals in your brain also play a very huge role as you might not be wired to enjoy depressants.
Also, Cannabis species also factor in. Most people are ignorant of this fact, but narrowed-leaf Cannabis sativa (native to the Indian subcontinent) has high amounts of THC and is the chemical that gives you the "high" affect when using marijuana. However, wide-leaf Cannabis indica (commonly referred to as hashish or hash) contains higher amounts of CBD and tends to give users more of a "stoned" affect as oppossed to "high".
Many people grow hybrids of the two species to mix the affects, but CBD actually can cancel out the affects THC. So, even if you smoke it right, mixing hash with weed will neutralize the affects.
Yet, hash isn't that popular here in this country and is more so in the Eastern hempisphere. However, I've heard people refer to resin as hash before, but that would be a misnomer. Hash is straight up a smokable drug like marijuana, but is a different species of Cannabis.
NOTE: Marinol (aka dronabinol) is a prescription form of THC. You can actually get it from your local pharmacy with a prescription from your doctor. Sativex (mouth spray) is also another medical variant of marijuana. They contain the same active ingredients as marijuana but contain none of the toxins.
As for info about marijuana's effects. Marijuana inducing psychosis is actually a fairly recent scientific find. Check out this CNN article obtained from the Associated Press in London.
NOTE: Marijuana IS addicting. Marijuana is a narcotic, meaning it causes sedation, induce sleep and relieves pain. Thus, due to it's physical affects which are felt in the body, Cannabis sativa can IN FACT cause physical dependency.
I have an article from the United States National Academy of Sciences. It explains in detail the health risks of marijuana.