He is a cool fighter who used to be on Lord Voldy-Mort's side but later saw the error of his ways and defected to Dumbledore's Freedom Fighters. He took up the position of Potions master at Hogwarts School of Witchcraft and Wizardry, and he lived in the dungeons. What a nice sounding guy!
He was constantly berating Harry Potter when he was at school. Snape killed Dumbledore as part of their plan to finally defeat Lord Voldy-Mort, as this made Voldy feel indestructible until Harry could kill him. Snape died in the final battle, he gave his life to make sure the plan succeeded.
And thus, everyone loves Snape.