I don't know why they fight so much. It's a real-life family feud.
Both Jews and Muslims are decendants of Abraham. Abraham's son Isaac is the forefather of the Israelites, while his son Ishmael is the forefather of the Muslims. Check this out if you want more info on that:
http://en.wikipedia.org/wiki/Abraham
So basically, Israelites feel like God loves them more because they come from Abraham's legitimate son, while Muslims come from the bastard he fathered with one of his servants.
And that's why they fight to this day. They both serve the SAME GOD (the God of Abraham) and even share many of the same holy scriptures, etc.
It's fucking stupid.