I am by no means an expert, but of course it boils down to Arabs and Israelis both staking claims over the same areas of land for centuries. That remained the way it was until 1967, when Israel did a land grab (and I guess spoils of war are legitimate, but...) still Israel hasn't budged since then but instead taken a strategy of creeping land grabs.
This in itself I cannot say if it is right or wrong since the intial encroachment happened against other states' armies (Egypt, Syria...) but Israel has continued to attack civilians and stateless people too, which is unacceptable in my eyes.