It's not only lust. It's lust and infatuation. As soon as one of those two factors starts to die, the sparks go away. But at some point, as with any high, the high goes away on its own anyway. If you smoke one joint a day for a while, eventually it's not going to get you high anymore. You're going to need more pot or a new drug. I should refer you to an episode of a Penn & Teller show I can't name because I don't like the way censorship works on this site. It's from season 2 and called The Business Of Love. You'll learn a lot more than I can tell you from people who know a lot more than me.