Aggression in fish, as in many other creatures, has many determinants. These may be environmental, biological, and/or social in nature. The Cichlidae family of fish, as a rule, readily display aggression in their natural habitat. This doesn't change in aquaria and is, in fact, exacerbated. If you're interested in learning the science behind cichlid aggression, there are plenty of scholarly papers on the subject, most of which are experiments conducted on aquarium fish.
Too many novice cichlid keepers have learned the hard way that cichlids can be brutal tank mates. Fish that look and act fine one minute can be beaten to death in less than an hour. Sadly, some irresponsible aquarists find cichlid aggression enjoyable, and hobby forums are rife with individuals who ask silly questions about which species is the toughest, etc.
IMO, two of the most effective ways to mitigate aggression (besides keeping a single specimen or none at all) are to keep very few cichlids in a very large tank or to keep a large population of different species in a modest-sized tank. The advantage of the former is space/distance and the advantage of the latter is redundancy. Giving cichlids plenty of room to either avoid tank mates or not interfere with a territory is usually effective. On the other hand, keeping many specimens in one tank distributes aggression and often prevents one fish from bearing the brunt or prevents any from staking out a territory.
There are other more subtle strategies to minimize aggression in your tank so I encourage you to become informed about these various methods before unnecessarily creating a behaviorally toxic environment for your fish.
The Cichlid Room Companion
Tropical Fish Hobbyist
American Cichlid Assoc.
African Cichlid Hub