Commentaire sur L’Exode 2:20

למה זה עזבתן — WHEREFORE HAVE YE LEFT [THE MAN]? — He recognised that he was of the offspring of Jacob because for him (Moses), as for Jacob, the water had risen in the well at his approach (Midrash Tanchuma, Shemot 11)

למה זה עזבתן?, since he is a guest, and a person who has demonstrated kindness you should at least have reciprocated by showing him a measure of hospitality.

למה זה עזבתם את האיש, "Why did you leave the man?" Yitro meant "why did you abandon a man who had done you a favour?" Alternatively, he meant: "how long can you let a man who has done you such a favour wait outside? Bring him inside, etc.!" Still another meaning of Re-uel's (Yitro's) words may be that though ordinarily Yitro would not want his daughters to strike up an acquaintanceship with men to whom they had not been introduced, this situation was different. He stressed that this man had already proved himself by his deeds. The word זה indicates that Yitro meant that this situation was different. This was especially so in view of the opinion expressed in Shemot Rabbah 1,32 that the waters in the well rose of their own accord towards Moses.