Funny how things work out. I think I know what happened at the Zoo and how Pink started to care about humans.

Remember these lines from the Zoo?

If we should wish to enter the containment area, we should do so using this service door! It's only been used once, for a human--

A long time ago, a Gem came through a wall-hole to help someone who was... "hurt."

It's implied that this hurt was emotional rather than physical (as the Amethysts didn't care when Greg was punched, but were sent in when the humans started crying. The humans even say "is this... hurt? Take this hurt away!").

A human was homesick and was feeling sad. Pink went in to see what was wrong. Pink learned more about how humans feel, and realized that humans need the Earth. This moment was likely what made her realize that she needed to preserve the Earth, and what sparked her transformation from bratty Pink to Rose.

Funny how things work out. I made a theory a while ago on Pink possibly being the one to enter the containment area. People replied saying that it wasn't Pink and was Rose instead, which made no sense. Now we know it was both, which makes everything fall into place. [Here's the theory if you're interested.](www.reddit.com/r/stevenuniverse/comments/8dfc4r/a_human_who_was_hurt_and_the_zoo_in_retrospect/)