Some folks smoke one cigarette and are hooked for life. Others smoke 2 or 3 packs a day for years and just drop them and never crave another. Same with booze and probably weed. We all might have a genetic predisposition that when it gets the right bait can hook us forever.
Genetics is undoubtedly a factor - but the fact is that of all those who have used marijuana, 9% have at some point been dependent, whereas the corresponding figure for alcohol is a substantially higher 15% (and 32% for tobacco).