@stoic_zee You said that either because you have a good relationship with your siblings or a single child. As someone who have a sister, these kind of things, is pretty normal. You could also ask your friend who have a sibling. I believe 60% of the times, they would answer that they often have a fight. But it's usually only happen until teenage years. Some continues until adulthood but more often then not, they both slowly mellows to each other. Besides, we fight not because we hate each other (most of the times). and the closer the age to each other, the more often the fight are.
But then again, it's depends on each individual. In this case, the kids probably imitate their parents at first and slowly creating a belief that "fighting is a way to show their love" just like how they parents even though they fight very often but love each other to the point of having children.
On the side note, it got me thinking... how are they on bed? do they still fighting, even when they are trying to conceive a kid?