Families argue and fight because every member of a family is human, and like all humans they are different.
The fact that a group of people live in the same house or grow up together doesn't mean that they're going to have exactly the same beliefs and views on everything, and it's because of this that families fight.
As children grow older and begin to develop a sense of self, they start to decide for themselves what they feel is right and wrong as opposed to what their parents tell them is right or wrong. Not agreeing with what your parents say is one way families fight and the children themselves also fight amongst each other because of this same issue.
A difference of opinion is probably the leading cause of arguments and fights in families and if each member doesn't learn to respect the others opinion then the fights will never stop. The truth is that every family fights, it doesn't matter how perfect the family seems, there will always be a disagreement here or there. What happens after these fights is what matters.
Fighting is a normal thing in families, it's something that should be expected. A lack of understanding and respect for each others opinions is often what causes fights in families and it's something that most families eventually overcome.
I hope this helps.