Three interrelated factors come to mind:
Quality: Obviously, not all the product that comes your way is good enough to melt your ass to the sofa. I remember the first time I smoked ATF weed ("Alaska ThunderF@ck") I thought I had found Paradise. For a while, I was disappointed that nothing else could get me that high. And then, through different connections, I found some really good melt-your-ass weed. These days, I don't see the point of smoking dirt weed; we grow enough good stuff around Seattle that I actually hear people complaining about Beaster (BC, Canada) imports. And this complaint is absurd; not only do our neighbors to the north keep us smoking when the cops are busting local operations, they do it with fine, kind product. That it's biker bud (partially compacted for transport via Harleys) is just part of what comes with the industry. (Mexican weed, by the time it reaches the Seattle market, is useless.)
Tolerance: You do build a tolerance to marijuana. Some of it, I would imagine, is a chemical thing; you'll build a tolerance to most drugs you put in your system. But some of it is ....
Permastone: Many people don't notice when they hit permastone. I missed it, and it was something I looked forward to. Perhaps my attention is the only reason I noticed it at all, but there comes a point when, even sober, you look at the world as if you were stoned. No, you don't walk around with permanently bleary eyes, or the smarmy stoner grin ironed onto your face, but your perspective adjusts, and the stoner's sense of absurdity creeps into your sober outlook. Things I used to overlook when I was sober now stand out as significant. Sometimes this can be the awful hair or fulsome words of a cable-news talking head; sometimes it's the grandeur of the clouds or the majesty of the trees; often, it's the humor of disaster. (Consider the old Carlin joke about the South American roller-coaster.) To the one, humanity seems quite alien from this perspective; to the other, it's easier to laugh instead of gnash my teeth when I hear President Bush's voice.
All these and more conspire to reduce the stony perception about smoking dope. My advice is to take a couple weeks off, and when you return to the market, make sure you're getting knockout weed. You'll notice the difference.
As a short note, consider environmental and social factors as well. I prefer to be stoned for concerts, but it's not the same experience as listening to good tunes with only a few other stoners around. To the other, if I were to hang out with a country-western crowd, or in a pop club spinning Top 40 remixes, I don't think there are enough drugs in the city to make it feel right.
:shrug: