If you do know people who smoke it goes both ways. There's a lot of people that marijuana really does just kill their desire to do anything productive (though I can say the same thing about videogames or a number of other things).
On the other hand, I don't know too many people where pot has had the reverse effect, and increased their drive. It does seem to have a numbing effect, broadly speaking.
Not saying I completely agree with the OP's sentiment, though.
Loosing the desire is second place. Excruciatingly, the ability is lost. It's a narcotic, mind numbing indeed. It has negative effects on aspiration, leading to decreased levels of oxygen.
Loosing desire is just a way to cope, switching one activity for the other, as both can't be combined. I am convinced the addictive effect, the positive connotation people allude to it are largely placebo.
On the other hand, I don't know too many people where pot has had the reverse effect, and increased their drive. It does seem to have a numbing effect, broadly speaking.
Not saying I completely agree with the OP's sentiment, though.