I think I finally get this song.
Here's what it means to me... (You don't have to agree, but it seems to fit to me
)
The 'manger' (especially in combination with 'stranger') is a strong reference to 'the stranger song' by Leonard Cohen ('He was just some Joseph looking for a manger') (if you don't know the song, you should find it, it's one of my personal favourites).
By using this reference he portrays himself as a restless soul, wandering from one safe haven to another. But when the safe haven turns into something real, something more, he feels compelled to run away. ("If love is not for fun then it's doomed")
He can't help making the same mistakes all over again, just like water can't help going down the waterfall (it even accelerates, when it gets closer), it's part of his nature.
This also changes the meaning of "i don't want to change you" into "stop trying to change me" (which is a bit passive aggressive)