I hate to be too simplistic about it, but I have three boys and two (soon to be three) girls and I can tell you that boys are SO much easier! :) Boys will duke it out and be done with it, but girls will stew and brood and pout and cry... Just the other day my five year old daughter was walking through the room and she just began to cry. I asked her why she was crying and she just sobbed out, "I don't know!"
What many people don't realize is that our bodies are going through hormonal changes even that young, even though it is years before our cycles will begin. Below is an article that outlines extreme cases of this, but the facts are that our diets and improved health and lifespans, not to mention hormone supplements added to our food, etc. have caused us to be able to reproduce much earlier that prior generations. This has naturally pushed back the emotional changes that begin several years before puberty. This is especially true for girls who are more emotional anyway.
Again, the article below has some interesting statistics on the onset of the hormonal changes that accompany puberty. I am not saying that your five year old is experiencing puberty, but the hormonal changes that must occur years before puberty start to surface surprisingly early, especially in healthy societies like the one in which we live. I hope this helps! :)
http://www.nexusmagazine.com/articles/puberty.html