I'm mostly a weekend party drinker and my drink of choice has always been beer. I tried moderation, but I found that despite the best intentions there were just too many times when I had that third beer and was just having a great time, I'd just say F it, and have another one, and then just one more, because I'm still in control, right, and then just one more and 'suddenly' all bets were off.
I realised that I can't say for certain how much I'll end up drinking once I start that first beer, meaning that the beer had more power over me than I had over it.
Also stopping after 3 beers was much harder than just not having any. Like MindfulMan said above, a constant negotiation. So tiring.