In many states, possession of marijuana is serious enough to get you chucked in jail. And underage drinking, especially underage drinking and driving, is a serious offense no matter where you are. Getting busted can ruin your life, not to mention embarrass the hell out of you. In fact every drug, in its own way, can be considered addictive. Pot is perhaps more psychologically addictive and habit-forming than it is physically addictive, but either way, it's hard to break free from using it.
Alcohol is absolutely addictive.
Most people think addiction is bad because it can ruin your life, keeping you from accomplishing what you want to accomplish, making you difficult to be around, screwing up your priorities, and hurting the people you love. Gateway drugs can easily do all of these things. Not everyone who uses alcohol or pot goes on to other drugs. In fact, most people who try a gateway drug don't get hooked, let alone move on to other drugs. But many people do, and it's not pretty.