Sure, no one needs alcohol to have fun (unless they've tipped the balance and are now an alcoholic, in which case they do and should get help). Nor does anyone need a cigarette to relax (see above for addicts). Drugs, ditto. If you want to be teetotal, good for you, it's healthier (although some red wine each day is good for heart disease) than the alternative.
HOWEVER - and capitals are required as it's a big one! - you're cutting yourself off from things too. There are certain experiences, positive and negative, that you're not going to have because you'll be too sober and sensible to take the risk or to embarrass yourself. Although not often pretty, getting hammered every so often is cathartic and is good for releasing tension.
Furthermore, even moderate amounts of alchohol can be lovely. A single glass of wine over dinner with good company can be fantastic, a cool beer on a warm day relaxing. The reduction of inhibitions as you get slowly drunk with a potential girlfriend is a great experience.
That isn't to say that you won't enjoy yourself if you don't drink, or that you'll never have great dinner conversations, relaxed afternoons or, indeed, uninhibited sex. But alcohol has been around for centuries, and is still around, for a reason. It can enhance a positive experience, provide fun you didn't know you could have and make relationships easier.
Remember, you don't have to drink to excess, you could just drink moderately without too many health risks and get the best of both worlds. I wouldn't choose to be teetotal, unless it was necessary for some reason.