He's afraid Israel might win. From the New York Times:
The Obama administration is increasingly concerned about the escalating violence in Gaza, believing that a ground incursion by Israel there could lead to increased civilian casualties, play into the hands of the militant Palestinian group Hamas and inflict further damage to Israel’s standing in the region at an already tumultuous time.If Cuba were to begin shooting missiles into Miami and Washington--even if they were only conventional weapons--I can bet that come hell or high water, the U.S. would respond with overwhelming force. It is racist and bigoted of Obama to suggest that Israel not similarly respond.
. . . A protracted escalation, the officials fear, could damage Israel’s already fragile relationships with Egypt and Jordan at a time when both of those governments have been coming under pressure from their own populations.