In the Predator Series we find out that the Predator species are hunters. They search for the most worthy prey, hunt them, kill them and collect them as trophies.
image src: What are the other alien skulls in the Predator trophy case?
What I'm interested in is, what makes humans worthy of the hunt?
In my opinion, we're weak (compared to the Predators), frail and not really much of a fight. Our only advantage is our brains (tactics), technology (including weaponry [warning: NSFW]) and our plot armour. Are those what make us worthy of hunting? Or do the Predators see something else in us which makes us worthy?
In Predator (1987) it could be said that they were just checking the place out. However they happened to come across one of our finest human specimens, Major Alan "Dutch" Shaeffer:
Image src: http://vignette3.wikia.nocookie.net/avp/images/3/3d/Dutch.jpg/revision/latest?cb=20101118172113
who was a truly extreme case of human, which happened to take one of them down. But we're not all Arnold.
In Predator 2, it can be said that we took that Predator down using our tactics and our strength in numbers (as well as our big guns).
In almost every single case of 1v1 fights against a Predator and a human, the Predator wins decisively. So as a single unit, we're not much of a fight or worthy of being hunted.