I'd like to think that after Simon and Grace get close enough to start inhabiting each other's space like people tend to, it hits Rocky with a sense of grief none of them were prepared for.
Not because he's losing his best friend or anything but because he can actually now understand the shape of Grace's loss, the enormity of what he gave up.
Humans fit together like puzzle pieces. Our hands fit together perfectly, our bodies find comfortable ways to curl up around each other. We even match the way we breathe with someone if we're close enough. Rocky has only ever seen Grace by himself and while he knows humans are a social species, obviously, he really only really has Eridian social behaviors to base that knowledge off of (Grace said movies are inaccurate to real human relationships but didn't explain how exactly). For Eridians touch is nice but it isn't like it is for humans, a whole system that goes all the way down to hormones. There's no way he could have understood how deeply baked in connection is for humans until he saw it for himself.
He finally has a reference for just how much Grace was missing and it tears him up to think about how close his friend came to just living like that forever. A single note in a song written for a choir.
If nothing else he is sincerely grateful to Simon for keeping Grace from that fate.
