The desire to "get the next high"? Marijuana is not physically addictive, unlike alcohol, tobacco, and caffeine. I've hung out with my share of potheads myself, and I've never seen anybody jonesing for a hit the way I've seen smokers jonesing for a cigarette. Or the way I've seen a caffeine addict stumble around bleary-eyed and uselessly until he's gotten that first cup of coffee in the morning.
I also know that when pot is not available, if they are offered something else, they often will accept it.
Ah yes, the ol' "gateway" argument. To whatever extent the gateway argument is true, it's true because to get marijuana, users must deal with criminals, who have good business incentive to introduce their clientele to more expensive, more addictive product.
Also, stupid laws breed contempt for the law.
When people discover they've been lied to about the deadly evils of marijuana, they may think that warnings against meth are also just 'crying wolf'.