"Your books betrayed you, Potter," said Severus, still in that voice stretched tight by a million tons of pull. "They did not tell you the one thing you needed to know. You cannot learn from stories what it is like to lose the one you love. That is something you could never understand without feeling it yourself."

from http://www.fanfiction.net/s/5782108/27/Harry-Potter-and-the-Methods-of-Rationality

Note: I'm not sure Snape did it, my instinct is that this is the work of QQ. But Snape has reasons.