Not exactly why the musician titled the song ":3" but I thought it was really cute.

I really enjoy this song this guy makes. And I enjoy a lot of his compositions of popular songs as well lol. He is very good. He improvised this song. It sounds really sweet, like, if someone who played piano were to write a song to his or her significant other, this song would be it. Okie, I think I'm done being all mushy and gushy about this.