Board Thread:General Discussion/@comment-26006155-20190820122909/@comment-38339538-20190904191424

It's also possible that Steven is able to fuse with humans only after he uses his Diamond powers to heal them.

Connie regained her eyesight and no longer needed glasses after she and Steven sipped from the same juice box.

Greg had his leg healed by Steven after he broke it during the confrontation with Lapis.

Both of these humans were healed with Diamond powers, and now have a part of a Diamond inside them. So Steven can fuse with them.