estrick: Black starts to go seriously wrong on his 8th move. If he had simply castled, he would have been fine. By moving his king knight a second time and then trading it off, he goes down nearly a whole point, according to computer evaluation.
On move 12, Black's best is to castle, with 12 ...Nf6 being rated equally good. 12 ...Nb6? takes the evaluation down from 0.82 in White's favor to 1.45 in favor of White.
Black then wastes more time by moving one of his knights for the sixth time in the game with 13 ...Nd5.
After White's advance of the c-pawn on move 15, Black is forced to move his knight yet again.
15 ...Nf6 would have been relatively best, with a computer evaluation of 1.76 in White's favor.
By moving to e7 instead, he's more or less completely busted, with the evaluation going to +3.37 in White's favor.