As has been said, it's complicated.
Is it because of UN resolution 181 which saw the founding of an Israeli state governing itself for the first time in 2000 years?
Is if because of the Balfour Declaration made by GB in WW1, saying the would give that small part of empire back to the Jews as a homeland, to gain their support in the on going war?
Is it the enmity between Arabs and Jews that goes back to Biblical times? The offspring of Ishmael and Isaac?
It is all of these things and more besides. Probably the longest ongoing conflict in human history.