He is a man filled with emotional damage caused by others. J.K made Snape as an example for people who have been hurt by others & instead of letting it go, they let the stupidity or the mistakes of other people infect them with bitterness. He loved Lily so much and it hurt him so much that she picked James over him. So, Snape is showing us that even when people are cold & bitter, it doesn't mean they want to be like that or that they were born that way, & it doesn't mean they are unable to love.
