Some things just are just a better match than other match.
As for to an example:
Coca-Cola and smoking.
The color red and the color green.
Grebigail and Grepenguin.
Rebigail and Repenguin.
Grebigail and Rebigail can also for to be a great match, although they fight a lot.
It's better to keep Grepenguin and Repenguin far away from each other at all time.
FIN.