Why so guys give us so easily when?

They pursuing a women. Why is it that they show so much interest at first then suddenly back off/ completely just become cold towards us. This has happened to me and my friends on several occasions. When they give up they're suddenly on to another female and he begins showing interest in her. This happened to me recently with a guy who showed a lot of interest in me and initiated things then after a while he seemed to get bored and just completely backed off. Now his with this other women. I know he is not trying to make me jealous but what gives?