1) You'll only ever have to move your own shit. You don't buy 600 pound couches or 12 foot long aquariums, but your friends do. And, eventually, they'll move somewhere with it. They're too cheap, or you're too helpful, whatever the case, you'll end up moving it. And hernias can last a lifetime, unlike most friendships.
2) If you're up at 3 am, it's because your own relative died. Eventually, if you are friends with someone for long enough, they are going to assume that you will support them at any time of day or night. And be it a drunk dial or a true crisis, your iphone/blackberry/rotary phone is going to start chiming at an ungodly hour at some point. Do you have many friends? This could be come a common, maybe even nightly, phenomena. And chronic sleeplessness can lead to accidents, a lowered immune system, and even death.
3) You can give yourself advice, and take it or leave it. Friends, once they have known you long enough, are bound to develop opinions about your life. It might be about your clothing choices, your relationship choices, or what body part to pierce next (belly button piercing for everyone!). And while at first it may be fun to have these 'suggestions,' you will soon be feeling pressure from your friends to make decisions that they feel are best for you. Your independence will be deeply compromised. And was our nation not built on freedom from tyranny?
There you have it.
Friendship: The Downfall of (Wo)Man.