The original funding (at current levels) came as a result of Israel deterring Syria from invading pro-American Jordan, as well as the decreasing French support (why support Israel if the French are already doing it?). The Cold War is over, so some of the original rationale for being generous to Israel changed, but some new ones have risen. Namely A) we want Israel to play nice with their neighbors, and giving them military assistance makes them less likely to feel cornered and lash out, and B) Israel plays a role in keeping the kind of people we really don't want in power from coming to power in their immediate vicinity. Might not be the best reasons for giving Israel a few billion a year, but it's not like we have a much better rationale for throwing billions at Colombia and Egypt.