1. Personalities - Some personalities just fit better with others and some take a little more time to work out the kinks. Some kids have an easy going get along with anyone personality and others can't stand anything that is not a particular way.
2. Age and Sex - Boys and girls are different and like different things and children of different ages have different interests as well. I have noticed that our 1 year old does not care if her brother who is nine gets a hug from mom or a song and snuggle, but gets very jealous if her 6 year old sister gets those things. I'm not sure if it is the age or the sex that influences that behavior, but we are working on it.
3. Sleep - When children are tired they get cranky just like adults.
4. Eating Habits - Sweets and things can definitely impact chidren's attitudes.
5. Parents Relationship - If parents fight with each other children are more likely to do the same.
6. TV, Video Games, and Computer Games - There is something about these and other forms of entertainment that seem to bring out the worst in people and kids are no exception. We seem to think the thing we are watching or playing is more important than the people in our lives and that causes problems.
7. Parents attitudes toward children - If parents are constantly to busy for their kids, griping at them, finding fault with them, and/or expressing feelings of frustration and anger rather than love; the children will do the same with their siblings.
I am sure there are more reasons, maybe you can think of one you would like to add. Sometime we will have to look at some things that can help reduce that sibling rivalry and help peace and love reign in our homes. I know I need help with that one!