In The Philosopher's Stone, when Harry is at the start of year feast, he looks at the teachers' table and sees Quirrell talking to Snape. Snape then looks past Quirrell and at Harry, at which point Harry's scar hurts him.
I was under the impression that Harry's scar was linked with Voldemort, and hurt him more whenever Voldemort was feeling particularly evil, or particularly close to him.
Is there a reason given why Snape looking at Harry should set off his scar?
It's not Snape that triggers the pain.
Professor Quirrell, in his absurd turban, was talking to a teacher with greasy black hair, a hooked nose, and sallow skin. It happened very suddenly. The hook-nosed teacher looked past Quirrell's turban straight into Harry's eyes -- and a sharp, hot pain shot across the scar on Harry's forehead
And guess who's in the turban...
EDIT
Some clarification regarding Harry's scar not hurting at other times he meets Quirrell.
First, Voldemort needs to be feeling strong emotion - this is confirmed in later books by Dumbledore. Secondly, while he is weakened and in spirit form, proximity is also necessary, hence his scar only hurting in the direct presence of Voldemort pre-GOF.
The events in this book that cause Harry's scar to hurt all involve scenarios in which we can imagine Voldemort is feeling a violent emotional jolt.
The scene the questioner is asking about is the first time Voldemort has "seen" Harry in person since he tried to kill him. Naturally, you can imagine he would feel some anger/emotional upheaval. When Harry stumbles across him in the forest Voldemort/Quirrell moves towards him, as if to attack - strong bloodlust/anger - a chance to kill the boy who lived! Finally, on physical contact with Quirrell in the finale - extreme pain caused by the protection curse. Probably some fear as well.
Is there a reason given why Snape looking at Harry should set off his scar?
Out of universe: So the reader fixes on Snape as the bad guy early on.
In universe: Because Voldemort was there on Quirrell's head, and at the moment Snape noticed Harry, Voldemort probably did as well.
Further, it's quite possible that his scar had been hurting mildly ever since he'd started being around Quirrell, but he didn't notice when being distracted by friends, and the fear of that new teacher's gaze may have made him painfully aware of the physical sensation.
It all makes sense if the only thing that makes his scar hurt is Voldemort getting angry. Perhaps when Snape looked at Harry Voldemort knew he was looking at Harry and saw love. Remember, Snape loved Harry's mom and Harry had his mother's eyes.
Plus this fits well with the story and how Snape was actually an inside man for Dumbledore and was actually an ally to Harry and Dumbledore but we don't know this until the end when once again Snape actually dies protecting him. In this way Snape is one of my favorite characters. Like a tragic spy who is cursed to live his life in the shadows of his misalliances, love that could never be, and torn emotions of love and hate in equal measure. A true, tortured / kindred soul.
