I'm a friendly single guy, but when I talk to girls they act as though I'm making them uncomfortable. How can I flirt with strangers without coming off like a creep?
Flirting is a dance between the flirter and flirtee. If you have two left feet, it's time to learn a new step. The fact that women are consistently giving you feedback that says "back off" doesn't necessarily mean there's something wrong with you, just your approach. Try getting an unbiased assessment of your pickup style.
Ask a couple of trusted female friends to observe you in action the next time you go out, then listen to their critiques. Women know what women like, and your friends can help you identify exactly what you are doing that is turning the ladies off. Don't take their comments personally—use whatever information they share to improve your moves.