the song is about a person who has someone that’s really special to them, However, it does seem to be a little dark when they say “I don’t think I’m ever gonna let you leave.” This does show how someone would feel selfishness, trying to keep someone all to themselves and only spending time with them (hence the, being in a treehouse and only allowing the person they want to stay in it). The song frames this in a really innocent and friendly way, as treehouses are usually associated with fun and childhood. It masks the darkness.