“‘I told you to empty yourself of emotion!’
‘Yeah? Well, I’m finding that hard at the moment,’ Harry snarled.
‘Then you will find yourself easy prey for the Dark Lord!’ said Snape savagely. ‘Fools who wear their hearts proudly on their sleeves, who cannot control their emotions, who wallow in sad memories and allow themselves to be provoked this easily-weak people, in other words-they stand no chance against his powers! He will penetrate your mind with absurd ease, Potter!'”
J.K. Rowling “Harry Potter and the Order of the Phoenix”