They experience such… difficulty. Yes, in their wars, their plagues, catastrophes, and the like. But the little things, those are what convince Castiel that humanity is in a state of perpetual suffering.
Eating. Sleeping. Walking. Defecating. It’s all so repetitive. The Winchesters excuse themselves at least four times a day to simply urinate. Dean was in the small washroom for an hour just fulfilling these tasks that seem to produce no results other than the comfort of familiarity.
Humans survive these setbacks through invention, and while they have accomplished much, their creations are still just a small step forward. Vehicles are begrudgingly slow. Their buildings are confining. They have no wings, so they build cold, stale imitations. Humanity is suffering.
To cope, they love. Endlessly. It’s greatly moving, seeing them engage. Castiel can’t quite understand how some love works. Familial love has always made sense, with his thousands of brothers and sisters. Companionship has naturally come through necessity. But the love that seamlessly binds two souls, so purely… he has always been stuck.
He asked Dean about it in the bunker one day. Perhaps his phrasing wasn’t ideal when he asked him if he could show him what true love is.
Unfortunately, Dean was drinking coffee when asked. He spat on the desk, over scattered papers, ancient texts. After much cursing and searching for paper towels, he finally sat back down, a strange look in his eye.