Shawn Mendes - Never Be Alone Lyrics Meaning

It is about about a boy who is really busy with work or stuff in general that requires him to be away. The girl and him have something important they have to talk about. The girl clearly doesn't want him to leave, but he has to, but he says, "take a piece of my heart and make it all your own." So he'll be with her whenever he isn't physically there. He later on says, "when you fall asleep tonight, remember we lay under the same stars." This means though we are far, I will always be there for you.