All I Want Lyrics Meaning

I believe it can be interpreted as two ways either as a death or a loved one.

sometimes when you lose a loved one you can get almost annoyed at them “why’d you leave me?”
Also an indicator for me that it’s about death is the fact he acts like he will never be able to see them again and would do anything possible to see them again.

However, after a breakup it can be very similar to them dying if you completely cut off contact with your partner as it will feel like you’ve lost a large piece of your life.
Also, the line “when you said your last goodbye, i died a little inside” could show that large part of their life they have lost. And the line “you brought out the best of me” can show how you can grow completely and become a different person because of them and now they feel like they have lost that part of them self or lost their old self because of the relationship.
I believe it is a beautiful and simple song which means it can be left to interpretation so each listener can listen to the song in anyway they want.