I wonder if someone could explain to me why my paint work is peeling?

I know this is the latest in a catalogue of "disasters" - but this is not
life threatening.

The paint has been on the doors ( two internal ones) for fifteen years at
least. I am not good at painting with gloss.

Recently it started to peel off on these doors. A small bit first and now
whole chunks. If touch it I can literally pull strips of paint off the door
leaving the coat underneath visible ( its green so I can see the difference)
..

I have considered just scraping off the top white gloss coat and leaving the
pale green ( except I hate green) in its place to save painting. But I
wonder why after all these years its just decided to peel off this way? I
am guessing it was painted green with apple white walls and woodchip paper
and even the obligatory avocado suite in the bathroom back in the 1970's/
early 80's?

If I re paint will I need to strip it all back to the wood or something?

Thanks for any advice.