| At first glance this would seem to be yet another drug-user-specific urban legend, like strychnine in LSD or rolls made of smack. Given some closer examination, and combined with first hand experience if that's your thing, it turns out that there really is some factual basis behind it. There are two main factors that contribute to this effect:
Content. Because marijuana is a plant, the chemical composition of its dried final product varies across an insanely wide spectrum. That is, factors such as breed, soil contents, growing climate, growing technique, final processing, and so forth all contribute in one way or another to what is in the drug when it arrives on the streets. Given that there are somewhere between ten and one-hundred (a full assay hasn't been performed, to my knowledge) psychoactive chemicals active in marijuana, and given that there will be widely different concentrations of these between any two batches of product, the two having different effects is not surprising.
Put together, if you are used to smoking from your own stash, and have built up some tolerance to it, smoking from somebody else's stash may present your brain with a set of psychoactives to which it has less tolerance. Thus, their stash may get you higher, QED.
Context. If you're smoking somebody else's marijuana, chances are that you are in a social situation with that person at the time. Perhaps it is even a situation that's fairly novel, like a house party or visiting a friend of a friend who you don't know very well. The important part is that the set and setting are different than they are when you ordinarily smoke up -- i.e. instead of thinking about The Simpsons while sitting next to your roommate, you're talking with some random person else in their bedroom, or whatever. These novel stimuli work together (at least for some users) to make the entire drug experience seem quite different from usual, which along with a differing physiological reaction may make the experience much, much more powerful. | 
