What ally does not represent their self interest first? Your question really isn't relevant considering all those who align themselves represent their own interests and become allies in support of their own aims.
"What ally does not represent their self interest first?"
And if the US' Commander-in-Chief determines that certain specific Israeli actions may NOT be in America's best interests, he can/should say so - and act accordingly.
PARTICULARLY since so much American Blood and Treasure has been expended in ways that benefit Israel.
Israel has reaped far more benefits from that relationship than it's sown.