Smarting over his unrequited love for James Potter.Snape didn't hate James because he secretly wanted him and was spurned; he hated James because James was a spoiled, privileged bully. He hated James not because his heart twisted with unfulfilled desire at every agonizing look at his beloved; he hated James because James was handsome, well liked, and got away with everything, while Severus was ugly, greasy, and unloved by even his own father.
James was the wealthy pureblood golden boy; heralded house team Seeker, protected by the Headmaster, Big Man on Campus.
Severus was a slimy half-blood Slytherin; lower-middle class, unkempt, bookish and sour.
It wasn't thwarted passion that stoked the fire of Snape's antipathy, it was resentment.
Really, enough already with the Snape-loving-James thing.