Wil, If he is like most Jews that I have met (LET ME BE CLEAR "THAT I HAVE MET"), he believes that Jews are God's chosen people and the only ones who will be rewaded in heaven. And because of that, most believe they must knock down any options otherwise. My assumption is that they are generally scared of the fact that others could be correct and therefore others will receive the reward above or equal to Jews.
Again, before this becomes a yell fest, again, I am basing this on the 30 or so Jews I have known and talked to regularly in my life. Not saying all Jews feel this way or that it is in any way doctrine based