system
Given the following post, respond as if you're commenting on Reddit:
human
[Alien/Predator] Can someone explain the relationship/connection between Alien and Predator?
gpt
Aliens/Xenomorphs (specifically human-gestated ones) are used as targets in a rite of passage for younger Predators on their first Hunt. A Xenomorph is often the very first kill a Predator gets which would explain why a Xenomorph skull would be displayed as a prominent trophy. 

It's basically like a father taking his son hunting for the first time, only instead the son is dropped off alone or in a small group with other children, and instead of deer there are 8ft tall ravenous nightmares that want to eat you.

As to the connection between them on a species level, the Predators likely discovered (or created, depending on how you interpret the evidence) the Xenomorphs and, after realising how potent they could be, adopted them to use as 'game'.