1. You can't do it all the time.
Even when I'm in that mood where all I want to do is close my door and read a book, I can't always do just that. Life comes as a package deal with this little thing called responsibility. Quite frankly, sometimes I flat out hate it. Imagine you didn't have responsibilities: no work, appointments, social gatherings, you don't even need to eat or go to the bathroom. If that were possible at this moment I would probably go soak in a tub and maybe fall asleep.
2. You get relentless flak from family and friends.
Mother calls up on a Friday night.
Mom: "Hi honey, what are you up to?"
Me: "Not much. Just playing with my fish".
Mom: "Oh... that's nice. Why don't you call your sister or brother and go out"?
Me: "Eh, I've already got my pjs on".
*10 minute lecture ensues about the need to make friends and socialize*
3. You start to get weird(er).
4. The chance of dating drops dramatically.
There isn't a whole lot to say on this topic. It must be common logic. The only way you would find someone during an extreme bout of antisocial-ness would be if you spent your time...no, if perhaps they, no... that won't work either. Maybe if you could...no, not possible. Sorry. At that point there isn't much hope.
I would go over the positive sides of it, but that would take up my whole afternoon. :)