Throughout the show we have seen different characters fusing. It would present that if you fuse, so does your mind. The two (or more) gems that fused feel stronger. When separated though, they feel as if their mind needs to be "whole" again, so they desperately want to fuse back together. But what kind of extent could this lead to? As you can see, gems like Jasper, for example, would become extremely desperate to fuse with another gem. So far to the point she fused with a corrupted gem. After Jasper was fused with Lapis for such a long time, Lapis' overall attitude changed a lot, also, this may lead to another theory. Lapis gone through a lot of tragic things, repeatedly becoming a prisoner for thousands of years, may have done something to her mind. When she was fused with Jasper, the aftermath showed that Jasper has become mentally unstable, and extremely desperate for fusion. This can be shown in other characters too, at different levels. For example, Pearl betrayed and lied to Garnet, just for fusion. Well, i'm gonna stop this theory here, because i'm kinda bored. TELL MY STORYYYYYYYYYYY
Fusions need to either be synced with one another mentally to maintain control or throw a Pearl into the mix. With the example of Garnet and Amethyst fusing there was a loss of control. Pearl was needed to keep them from losing control. I have a theory that Pearls have a quality that keeps fusion from losing control since all Pearls seem to be about having the attitude of maintaining order, control and such things.
If Pearls were in fact from White Diamond in a way it could make sense they picked up that personality from White Diamond of being orderly.