20 Things You Didn't Know About The Xenomorph

Original director Ridley Scott believes the typical lifespan of a Xenomorph is only a few days. Like a Mayfly or a Drone Ant, Scott claims the Alien exists only to further the existence of its hive, before giving up the ghost entirely and heading to Xeno-heaven.

So, at the end of the first movie, when Ripley and the lone Alien are aboard the Narcissus, the creature seemingly leaves her alone because it's content that it has found a nice quiet place to end its days. Only when Ripley provokes it does the Alien attack. This is not canon however, and we'd wager it was actually just hiding out to ensure the survival and continued growth of its species.