Why does David feel that he's ungrateful?

David feels he's ungrateful because Mr. Campbell was so obviously worried about him and sad to see him go.... unlike David, who can't wait to get out of the countryside. 

With that he got upon his feet, took off his hat, and prayed a little while aloud, and in affecting terms, for a young man setting out into the world; then suddenly took me in his arms and embraced me very hard; then held me at arm's length, looking at me with his face all working with sorrow; and then whipped about, and crying good-bye to me, set off backward by the way that we had come at a sort of jogging run. It might have been laughable to another; but I was in no mind to laugh. I watched him as long as he was in sight; and he never stopped hurrying, nor once looked back. Then it came in upon my mind that this was all his sorrow at my departure; and my conscience smote me hard and fast, because I, for my part, was overjoyed to get away out of that quiet country-side, and go to a great, busy house, among rich and respected gentlefolk of my own name and blood.

"Davie, Davie," I thought, "was ever seen such black ingratitude? Can you forget old favours and old friends at the mere whistle of a name? Fie, fie; think shame."