My best memory of my youth---but it turns out to be a sentimental thing

I still remember that afternoon :
my first boyfriend and I went to a hill beside my campus
after getting there , i started to draw a picture
he climbed on a tree beside me and watched me drawing
once i took a break,

I saw him in the tree, and the warm sunshine was behind my prince
"what a beautiful sight!" I thought
at that moment , I felt full of hapiness
several years later , I always remember that image and ask myself:
why can not it stay forever?
why did we break up, since we were in love with each other at first ? (this sentence is not so clear to me)