Nechama Leibowitz has an explanation that (IMHO) beautifully combines peshat and midrash. She notes that, leading up to Moshe's prophecy at the bush, there are three progressive stories recorded about him.
The first is the story of him seeing an Egyptian oppressing a Hebrew, in which he kills the Egyptian. This demonstrates how strongly he felt about saving the oppressed, but one might think that he only got involved because he was defending a member of his own people against the enemy. Next, however, we see him getting involved in a fight between two Hebrew men, showing that Moshe was a man who wanted to stop fights even within his people. Finally, we find that after he runs away, he defends the daughters if Yisro from the shepherds. Despite the fact that these women were perfect strangers, Moshe's sense of injustice was so strong that he was ready to assist anyone who needed help, no matter who they were.
The midrash (Shemos Rabbah 2:2) picks up on this trend, by developing it a step further: Moshe was tending to Yisro's sheep, and was willing to help even a single lamb get to the sustenance that it needed. The point that the midrash is bringing out relates to this portrayal of Moshe as a defender of the weak and righter of injustices, a man who deserves to lead a people.