I never have really been able to figure out the Left's dislike/opposition/hatred for Israel. Israel is democratic, tolerant, etc...there are even Arab Muslims in the IDF. On the other hand, their Islamist fundamentalist adversaries are intolerant religious fanatics who oppress women, treat homosexuality as a death penalty offense, think nothing of killing women, children and the elderly - in other words, they actually are everything Democrats accuse Republicans of being.
The only things I can think of that make sense are that leftists like to think of themselves as standing up for the "little guy"/underdog, and the Palestinians (and other Arabs) qualify for this role because they've had their asses handed to them by the Israelis on a regular basis.
Then there's the historic, Cold War vintage ties between various Palestinian militant groups, Arab countries, folks like the Red Army Faction, Baader-Meinhof, IRA, etc, all of whom were used by the USSR in various ways against the US and the West.
Or maybe just old fashioned anti-Semitism. A Progressive tradition since, well, the beginnings of Progressivism.