Musical theatre is amazing. If you ask someone to define "theatre music", they wouldn't be able to do it. But give that person a song and they will immediately know if it's from a musical or not.
I've lost count how many times I've sat in the theatre listening to a musical song with goosebumps all over my body, or where the songs made me want to jump from my chair and dance with the ensemble.
With that same passion, I write my own musicals (script, song book, lyrics, everything).