"...cannot afford mistakes, Draco, because if you are expelled--"
"I didn't have anything to do with it, all right?"
I hope you are telling the truth, because it was both clumsy and foolish. Already you suspected of having a hand in it."
"Who suspects me?" said Malfoy angrily. "For the last time, I didn't do it, okay? That Bell girl must have an enemy that no one knows about--don't look at me like that! I know what you're doing, I'm not stupid, but it won't work--I can stop you!"
There was a pause and then Snape said quietly, "Ah... Aunt Bellatrix has been teaching you Occlumency, I see. What thoughts are you trying to conceal from your master, Draco?"
"I'm not trying to conceal anything form him, I just don't want you butting in!"
Harry pressed his ear still more closely against the keyhole.... What had happened to make Malfoy speak to Snape like this--Snape, toward whom he had always shown respect, even liking?
"So that is why you have been avoiding me this term? You have feared my interference? You realize that, had anybody else failed to come to my office when I had told them repeatedly to be there, Draco--"
"So put me in detention! Report me to Dumbledore!"
~Half-Blood Prince, Ch. 15 "The Unbreakable Vow"
That might not be the best place to stop, but I'm tired and don't feel like staying up much longer. Anyway, I think that, after the Spinner's End chapter, this is the hugest scene that really makes you decide your opinion on Snape's loyalty the first time reading the series. It either gave you even more of a concrete reason not to trust him, or made it even more obvious that he was loyal to Dumbledore.
And as for Draco: personally, I don't think Snape ever really liked him. I think Snape would have resented any contact with the Malfoys, since Lucius is the one that presumably suggested him to the Dark Lord, and was also still loyal to Voldemort even after his supposed death. Plus, Draco's a mini James Potter, minus the fact that Harry's the one who looks like James. So I think Snape merely acted as though he liked Draco, because otherwise, it would have been suspicious.
