why do dogs always fight?

Answer 1

Dogs do not always fight.
The biggest reason some dogs fight is to gain dominance, or position of the "alfa" male or female.
Neutering and spaying greatly reduces the instinctive desire some dogs have to gain this position, but dogs that are dominate in nature may always exhibit these traits, at least to a certain point.
Their environment is also a facter. Food competition could play a part as well. It has been my observation that the more dogs in the family, or the bigger the "pack", the more likely there will be challenges for the alfa position. They may also challenge their people for this position as well, although most domestic dogs accept their person as the alfa.
Dogs fight over territory, but again, this is a dominance thing.
They may also fight when they feel threatened.
The way dogs are raised is also a factor. Sometimes it is nothing more than lack of obedience that cause dogs to fight.
Not all dogs have the dominace trait. Many dogs are submissive and are not likely to fight except in defense, and alot of those dogs will not fight even then.
So, your question doesn't really have an exact answer. If you have real interest in this subject, you should research the behavior of dogs. If you are having problems with your own dogs fighting, you definetly should research the subject. Maybe there are some things you can do to stop it, but sometimes separation is the only way. There are canine behavior specialists out there who could also help.

Answer 2

They don't always fight but when they do it can be for several reasons- male aggression due to unneutered males and competition,personality clashes,female vs female "competitive" aggression,illnesses,territory,food....It's natural instinct for canines to fight for their positions in the pack order also. Fighting is instinctive for survival in canines.