First and foremost self defence. Secondly, exercising sovereignty over lands abandoned by Jordan, not transfered by Jordan to a non-existing entity that today has been cobbled together as Palestinian People. Not that there weren't Palestinians but there was not polity of Palestine. Abandoned lands, under international law, are subject to the first in time to exercise sovereignty.
Spain investigating ex-IMF chief Rato for fraud (Reuters)
from the article: Hugo Chavez: Israel plans to 'terminate the Palestinian people'