Although, I still find it really funny that Snape can keep his emotions hidden from Voldemort, but has a harder time when Harry's around. :)
That's always been a conundrum... I reconcile it to myself by imagining that V is is not assaulting Snape through his emotions, but through his mind. ie: Snape is adept at hiding and guarding an emotional weakness with V because his emotions aren't truly engaged during their interactions--he's guarding his thoughts of emotions. (With the possible exception of Snape not able always to hide the emotion of fear, which might actually be a great diversionary tactic for him to employ since V would expect and celebrate that emotion!)
With Harry however, Snape is assaulted by Harry's own emotionality and is swimming in feelings which he has never mastered. He is kept off-balance by his visceral emotional response, and his intellect is never fully in charge. Harry lives in, evokes, and operates from, a world of emotional, rather than intellectual realities, and Snape is ill-equipped to manage what he most likely considers an inexplicable attraction. (Plus, he's made repression an art form. Mt. Vesuvius comes to my mind! lol!)