Watching the flowers grow and feeling love in your heart are two prime examples of how beautiful the world can be sometimes
Falling out of love, being hated for no specific reason, watching a loved one die, and being lonely are a few prime examples of how cruel the world is sometimes
There are many other examples for both of those and I can't think of them off of the top of my head
I've always heard the term "the world is a beautiful/cruel place sometimes"and I was always prepared to experience both
I've had my heart broken, I've watched something/someone die, I've been lonely for the longest amount of time I can think of... But those are just the cruel things
I've felt loved, I've watched the flowers grow, I've watched the sun rise at 6 am, I've felt less lonely surrounded by people I can call my friends... And those are the beautiful things
Keep that term in mind, "The world is a beautiful/cruel place sometimes"