Why did Eliezer attribute his refusal to his success?
Rashbam: Because Hashem did not want the matter delayed, and if there would be a delay, it would not be a total success.
Ha'amek Davar #1: Because if a messenger fails in his mission, he is pleased if there is a delay, so that the sender worries what happened to him on the way, and when he ultimately returns, he is happy that he returned and is not so pained that the mission failed. But if he succeeded in his mission, there is no reason to delay.
Malbim and Ha'amek Davar #2: Since Rivkah came immediately, miraculously, this shows that not only the match was from Heaven, but also that Hashem fixed the time immediately, 1 and to remain any longer would be delaying the time that Hashem fixed.
See, for example, how he had arrived in Charan on the same day that he left Chevron. (EC)
Why did Eliezer repeat, "Shalchuni lA'doni," but he added the word "v'Elechah"?
Ha'amek Davar: "Shalchuni lA'doni" includes if Rivkah would leave their house and remain in the city (e.g. a year or 10 months), and Eliezer would guard her there. He replied that they should send him to return immediately.
