- "I don't like losing." No one does. Use it as an opportunity to learn from your mistakes. Dan Heisman wisely observes in Everyone’s 2nd Chess Book "You'll more likely learn from losing [than winning] as long as you identify the cause of the loss and make a positive correction for the future." And keep things in perspective - the important thing about winning and losing was best stated by Vince Lombardi: "Winning is not everything, but wanting to win is."
- "I don't know the chess openings well enough (or the middle game, or the endgame, whatever)". Nobody does - except maybe Deep Fritz ;-) I think a lot of the fun in chess is discovering what you don't know for yourself - when your opponent either does something clever or punishes you for doing something stupid. Yeah, sure you could learn all the traps and tricks from books, videos, and chess software - but it is much more fun to learn them the hard way across the board from a fellow player.
- "My rating might go down." So what? Chess is supposed to be a recreation - an enjoyable recreation - if it isn't why play at all? Don't spoil the game by obsessing over a largely meaningless number. Your rating is simply a tool, which it seems to me has two purposes 1) as a tool to help in pairing up players of similar strength 2) and, possible, as measurement for tracking one's chess progress.
So, get out there and play - no more excuses! Enjoy the world's greatest game! This is supposed to be fun, right?