What are some reasons someone would be 'hot and cold'?

(this is just an example)

You meet someone, you hit it off. They are single, as far as you know, and obviously interested in you, maybe they are even overt about it. They get you interested back, and you flirt and have fun. You think things are going somewhere. Then, with no warning or reason that you know, they disappear or act cold or generally behave like the ‘hot’ exchange never happened.

You’re confused, but manage to shrug it off and go back to your life, perhaps a bit disappointed. Then, with no warning or reason that you know of, the person jumps back into your life and starts flirting hard and making their interest apparent once more.

This kind of thing perplexes the hell out of me. I mean, you’re interested or you’re not, right? Why would someone engage in this ‘hot and cold’ behavior, especially over and over again?