They like the aggression I guess.
I wish that fish only grew to their tank size... I would have hundreds of really cool fish in my house but unfortunatly is just isn't how it works, I'll use an RTC as an Example this is a fish which ALOT of us here own and that alot of people here will and have to get rid of because they simply can not house it in a big enough tank... if fish grew only to the size of their tanks then I could see alot more of these guys being kept for life in 150-200 gallon tanks.
There are infact ways of "stunting" the growth of fish however when you do this you are comprimising the quality of life of the fish as well as stunting it's life span as well... it's really inhumaine in my opinion to do such a thing. it's like stunting the growth of a child... just becasue you don't want to buy them bigger clothes...doesn't make sence to have the kid if your not willing to take care of it properly