Alcohol a very different drug and should not be compared with cannabis, though it almost always is these days. You either drink or smoke.
Getting drunk is much like throwing a party. You invite all your good friends round who you've known for years and serve food you've had before, but have lots of fun and the next morning there's a big mess to clean up.
Getting stoned is like going to a party being thrown by someone you've never met before. It's a friend of a friend, but you don’t know many people there and though it might not be a guarantee you'll have as much fun as if you're with your mates, it's interesting and you learn new things; meet new people.
If you drink, there's only one thing that's gonna happen. You get drunk. If you smoke pot, anything could happen. Maybe nothing at all. Probably not much. But you could wake up and see the world differently. (If you smoke a lot you'll see life get more hard work until you do nothing but smoke pot).
Basically, alcohol is a poor alternative. Marijuana is something you share in reality and mentality, and it can often form the basis of friendships. Having said that, there are plenty of people who do see it as a perfectly good alternative; smokers and drinkers almost always mingle. There's a huge moral dispute over alcohol of course eg: More dangerous, more deaths connected with it etc., but they probably just want to see you having a good time in the way they know best.