Severus suffers in his youth from the curse that most teenagers (and adults) have endured at
one point or another: not fitting in. His Muggle father and pureblood Witch mother fought,
and home was not a happy place. Through some relation or other resource, young Severus is
introduced to the Dark Arts. He is intrigued and finds something that he discovers he doesn't
want to live without: a sense of control. He goes to Hogwarts armed with a defensive personality
and a lot of knowledge about curses and hexes for one so young. Severus has learned from
watching violent and unsettling scenes between his parents that while love may be a commodity
you cannot force someone to give you, there are ways to compel respect. He is sorted into
Slytherin House. From what the Sorting Hat tells Harry during Harry's own sorting, this indicates
that Severus, as early as age eleven, possessed not only a heavy dose of survival instinct,
shrewdness and cunning, but also the desire to be great. We know from Sirius' comments in GoF
and OotP that Snape was already very well versed in curses and the Dark Arts when he came to
Hogwarts. Yes, Sirius was biased. But Sirius' hatred of the Dark Arts and seeing that very
developed tendency in his classmate might have helped create that bias. We know from Harry's
journey into Snape's OWL memory that he was not popular. We know from his Potions textbook that
Harry used in HBP that he was a diligent student who seemed to work hard, not only at improving
the potions in Borage's book, but also (as Sirius indicated) in creating an assortment of hexes
and curses that ranged from the mundane to the lethal.
The thing about growing up with Fred and George is that you sort of start thinking anything's possible if you've got enough nerve.
Ginny Weasley Order of the Phoenix, Chapter 29, Page 655
So many fans visit King's Cross station to take pictures of platforms 9 and 10 that the station management erected a sign that says 'Platform 9 3/4 which, in the Potter books, is invisible to Muggles but acts as a gateway for witches and wizards.