That's something I don't understand.
A lot of my friends who drink... only drink occasionally. They don't always buy beer to drink at home every night. When I come over to visit, he will share a bottle or two with me while we sit and talk and watch movies or play games. When we go out to dinner or a bar, we'll have a drink or two. They don't come home, go straight to their fridges, and start drinking the rest of the night away as if it's more important than everything else.
I do have a few friends who are serious alcoholics... so bad that it isn't funny. It has resulted in nearly burning down apartments, getting in serious accidents, and going to the hospital several times. Even after these things happened, they don't change... they just drink more and more.
That's one of the reasons why I don't talk to them much any more. Whenever I see them, drinking is the only thing they ever talk about. "Hey man! How are you? We should go to the bar to drink!" "Yo, what's up? I'm having a party on Friday. Let's play beer pong!" "Hey, you graduated? Cool, this calls for gulping down a bottle of vodka!"
Even if I try to bring up something different, they go back to talking about alcohol. "Hey, there's a good OC movie this week. Wanna see it?" "Umm... I guess. Let's play beer pong afterwards!" "I'm going to meet my friend for dinner. Wanna join us?" "I dunno, will there be beer?" "I'm going to a party. We're gonna play Monopoly and watch a couple movies." "Cool, I'll bring the beer!"