A nobody was someone never meant to exist, so perhaps that was why as nobodies they would remember the lives they had lived before. It was almost a taunt showing them what they couldn't actually have anymore. Showing them what they had lost but as an nobody they had no heart and no emotions so all there was a big empty void that could never be filled.
Zexion found that when he had truly nothing to do that was when Ienzo's memories tended to resurface. Which Zexion found he could care less about the whole situation. Any emotions that Ienzo felt, were Ienzo and his alone.
The worst was when the memories seep into his mind, when he wasn't paying attention. It was always a strong memory that went first, usually the memory invokving Xehanort. It seemed that Ienzo had carried strong emotions for the man.
Zexion however harbour no secret feelings for Xemnas because, he had no heart to feel with in fact he didn't feel anything towards Xemnas at all. Which really should have bother Zexion a lot more then it did. Ienzo had been so devoted and willing to do anything for his love. After all, that was how they got test subjects int he first place. Xehanort had convinced Ienzo to talk with Ansem the Wise.
Which had been the turning point for both Ienzo and Xemnas's hearts. Putting a quick end to that relationship.
A relationship that could never be built again because Ienzo and Xehanort were gone. All that was left was Zexion and Xemnas, two empty shells.