I'm probably going to get crap for this, but I see a lot of people saying things like "I only want this one girl/guy and nobody else (who may or may not know I even exist)" - that sounds like a recipe for disaster to me. What if you do get together with that person, and they turn out to be completely different than the expectations you have for them? For about 5 years now, my brother's had his sights set on a girl who's recently married, but he doesn't understand that he's completely incompatible with her. He wants to play video games just about as much as possible (taking the dog to the vet is an awful long time away from Steam...), and she's the "hollywood pretty girl" type, like her perfect day is shoe shopping and fancy expensive dinner, but he looks at her having a happy life and being pretty and thinks she'd make him happy.
For the record, I'm taken now, but I've spent plenty of time being both happy and unhappy when I was single. Dating before my current girl helped me learn what I really want out of a relationship, and that I was an idiot when I had a few relationships that involved me not being myself (i.e. things like hiding any nerdy hobbies like video games, and showering gifts on the girl)
If I can impart any advice on someone who's in the "single and unhappy about it" camp, don't lust after that one person who you think is going to be what dreams are made of. Everyone has faults, and many times we think we'll be happy with someone because we're happy and we see them being happy, but if they don't share your interests, you'll have one hell of a hard time being happy together. Be yourself, always, or you'll never be happy whether in a relationship or not.