Yisro told Moshe that he shouldn’t be the sole judge for the entire nation. There should be other judges helping him. The sefarim teach that Yisro’s point was valid. It was indeed impossible, according to the rules of nature, for Moshe to judge the nation alone. Yet, before Yisro asked that question, Moshe was able to judge the nation alone, because Hashem gives people strengths – beyond the rules of nature – when they want to do His will.
However, after Yisro asked his question, he brought the matter down to the realm of nature, and then it became impossible for Moshe to judge them alone. Now he needed aid. Therefore, Moshe sent Yisro home; his questions were weakening his strength. ~ R’ Elimelech Biderman Shlita