Leading girls/guys on

Many times in the past I've ran into problems with some girls because we talk or hang out and I'm only interested in them as a friend, but they end up catching feelings for me. I look back and realize I probably led them on, but I don't understand how to be nice and cool to somebody without them getting the idea that I'm interested romantically in them. Has it ever happened to you? Have you ever led somebody on and didn't know it?

Sometimes you need to declare that you want to be single or something first at times. For me personally I let them develop feelings for me but it won't stop me from hanging out with them. I will still treat them as a friend even though they like me.

True, it just gets uncomfortable after a while and then after I let them know I'm not interested they get even more clingy. Note that these girls aren't very attractive and I know I'm fully aware that I friendzone them. Those are the kind of females I attract for some reason.