Definition For Drug:

1. A substance used in the diagnosis, treatment, or prevention of a disease or as a component of a medication.
Such a substance as recognized or defined by the U.S. Food, Drug, and Cosmetic Act.

2. A chemical substance, such as a narcotic or hallucinogen, that affects the central nervous system, causing changes in behavior and often addiction.

3. Obsolete. A chemical or dye.

Well it's not just the quality of the weed, although the good shit is ideal. It also depends on your environment at the time, whether you're huddled up outside in the cold trying to kill off a roach, or hanging out with some cool ass people casually passing a bong around. It may also depend on the materials your using. For example, I'm partial to blunts, and right now I have an assortment of some deliciously flavored wraps. I've even got one that tastes like chocolate chip cookie dough, and I have a more enjoyable high when I've got a pleasing taste in addition to that of the herbs.

Anyways, I tend to see it as the better mood you're in while high, the more creative you are.