Why Hold Back?
Moses said to Aharon, "Draw near to the altar." (Leviticus 9:7) Rashi wonders why Moses needed to tell this to his brother. Surely Aharon knew what his duty was. But Rashi explains, "Aharon was hesitant, so Moses had to say to him, 'Why are you hesitant? This is what you were chosen for!'"
Rabbi Moshe Leib of Sassov offers an amazing comment on the commentary. Why, he asks, does Moses tell 
Stride ahead and do your duty.
Aharon, "This is what you were chosen for"? Most of us would interpret Rashi's words like this: "Go, approach the altar, for this is what God intended."
Rabbi Moshe Leib understands Moses differently: "Aharon," he believes Moses was saying, "So you are diffident? Fine. That is how God made you. Diffidence, modesty, humility - they are part of the make-up God gave you. You are diffident because that is what God wants. It is far better than being too conceited and over-confident. But you have to overcome the diffidence and stride ahead and do your duty regardless, and God will be with you."
To Make a Difference
The sidra ends with the duty "to make a difference" between the kosher and the non-kosher. In that original sense "a difference" means "a distinction". We have to know what we can eat and what we can't, and to have the moral strength to accept only what is permitted and to reject other things, however attractive.
In modern English, there is a secondary meaning to making a difference: living in such a way that our deeds make our society better and finer, leaving behind a world that is different, however much or little, because we were there. It doesn't necessarily mean that the whole of history is changed because of us, but that we help mankind to edge closer to its goal.
One way of defining that goal is to make the world a Gan Eden without the sorrow and suffering that human beings cause one another. We will get there eventually. Where Gilbert and Sullivan say, "My object all sublime I shall achieve in time," we prefer to say, "we - all of us together - shall achieve in time" - provided we all "make a difference".
A Rachmonus on Uzzah
The Haftarah tells a story of a good deed that went wrong and brought disaster. There was a dramatic procession led by David to bring the Ark of the Lord to a new resting place. But instead of being carried on the shoulder as in the wilderness, the Ark was transported in a cart made for the purpose.
Uzzah and Achyo, the sons of Avinadav, "drove the new cart" (II Samuel 6:3); this means, according to Kimchi, 
Instinct isn't always the best guide.
that they guided the oxen that pulled the cart along, with Uzzah walking by the side of the Ark and Achyo walking in front. However, when they reached the threshing floor of Nachon the oxen stumbled and Uzzah "put forth his hand" to steady the Ark. There was no vote of thanks. Instead, "The anger of the Lord was kindled against Uzzah, and God smote him there for his error, and there he died by the Ark of God." (verse 7; cf. I Chronicles 13:10-11) As they would say in Yiddish, a rachmonus oif Uzzah - "What a pity for Uzzah."
What did he do wrong? One view is that as a non-Levite he should not have touched the Ark. Another is that he lacked faith that, as the sages put it, "The Ark sustained those who carried it" - literally, "The Ark carried its carriers." We do know that after Uzzah's death the previous practice was restored with the Ark borne on the shoulders of the Levites (I Chronicles 15:15). David was indignant at the incident, in all probability because insufficient instructions had been given to those who handled the Ark (I Chronicles 13:11); they should have been warned that incorrect handling of holy things could result in death (Numbers 4:15).
Nonetheless, we feel sorry for Uzzah. He really meant to do a good deed. Did he deserve such a terrible fate? The Biblical text must be telling us that all one's actions, even the good ones, have to be weighed in advance. Instinct isn't always the best guide.