Millions and millions of reasons.
Presidential adviser Gary Cohn reportedly was so upset over the administration getting close with American Nazi's and White Supremecists that he thought seriously about resigning. But he didn't.
But now that the Taxer in Chief will raise taxes on imports, the guy quits. See, he stayed on to get a massive tax cut for the wealthy enacted, and Gary ole buddy is wealthy. So once he got his millions he could now leave a satisfied person. Some people will say he left his integrity at the White House, but that is not true. He had no integrity to leave.