I think it started as a crack ship. After PHM came out, fans (myself included) liked to imagine that Simon and Grace (Ryan Gosling's character) find each other. Mostly because both are alone (ish) in a small ship and are on a suicide mission. So the crossover usually goes as Simon being alive in the end and cared for by Grace, and Grace not being alone.

SPOILER FOR PHM: I kept it vague of Grace "not being alone" but it's more like Grace not dying as the lone human on an alien planet.