I have a lot of shy friends. They won't take the chance of approaching someone new for fear of rejection. I think rejection is the worst reason not to approach someone you're interested in. You'll never know what could have been If you don't extend the "peace token of your choice."
Guys, straight OR gay, seem to be the worst offenders. They decide that waiting for a person they're interested in to approach them is the best approach. It doesn't work... ever. You'll spend ten minutes watching them in full creeper mode and then... they walk away and you never speak to them and probably never see them again. And it's not usually because they never run into you again; it's because you stared for 10 minutes without speaking. Hello creepy stalker fellow.
People should try the "Hi, can I buy you a drink" more often. It works on me and I'm wearing a wedding band. If you offer to buy me a drink, I'll take it. I'll order it myself, have the bartender hand it to me so I don't get drugged (thanks for that lesson, Compound,) and spend a few minutes talking to you while YOU pay for it. You've got my attention, even if it's for just a moment.
It took me a long time to approach strangers but I'm pro now. Being forced to speak to strangers was terrifying at first but with a little practice and a realization that not everyone will bite, you learn to stop dreading the "thanks but no thanks" and start enjoying the people who do accept.
Of course, my "promo boy" gig helped.
And for god's sake, if someone's sitting alone, say hello! No one really likes being alone at the bar.