Severus froze at the sight before him. He had ducked into this classroom to avoid Black (who would have cursed him given the chance), and he had come across this. James Potter was snogging Lily Evans. Severus' jaw dropped in shock as his heart skipped several beats.
"If you could excuse us, Snivillus," Potter drawled, somewhat breathlessly, but haughtily all the same. A fierce anger boiled up in Severus at Potter's words. Subconsiously, he knew that Lily hated him after his 'mudblood' comment from 2 years ago. At that moment, however, Severus could think of nothing but his anger. How hurt he was that Potter was snogging his Lily. The nerve of him!
"Sectumsempra!" Severus snarled over the threat of tears.
"Wha-?" Potter was shocked at the sudden appearance of a series of huge gashes to appear all over his body. Lily screamed. That sound was all that brought Severus back to sense. He looked at Potter, horrified at the thought that he'd lost Lily again. He looked over at Lily, transfixed by the sight of her sobbing over Potter. He realized then that he couldn't lose something he hadn't ever had. She had never loved him. If she had, he had lost her long before this. She loved Potter. The look on her face as she saw Potter injured was the same as the one on his as he saw her cry. Severus' heart shattered completely. He had never hated Potter as much as he did at this moment, and never loved Lily more.
Tears running down his face, Severus ran over and healed Potter. He left a bottle of the Essence of Dittany next to him and walked off. As he left, he heard a soft voice calling his name.
"Sev?.." Lily whispered tearfully. Severus turned around and met her beautiful green eyes.
"I'm sorry, Lily…" He whispered scratchily, tears still falling.
"I know, Sev," she replied quietly, "you wouldn't have healed him if you weren't."
Severus bowed his head as the tears came faster and turned to walk away again.
"But Sev," the soft, hurt voice continued, "why?"
Severus met her eyes one last time. He drank in her face. The green eyes now full of hurt, confusion, and suspicious gratitude. Her pale skin, dotted with the lightest sprinkling of freckles. Her dark red hair, shining in the sunlight filtering through the dusty window.
"Because I love you, Lily," he said, looking straight into her eyes. Then he turned on his heel and swept out. Severus dashed quickly to another deserted room and sobbed. The only person who had ever cared for him was well and truly gone. Thankfully, today was their last day at Hogwarts. After this, no one would know. He trusted Lily wouldn't tell anyone.
After that day, Severus Snape wouldn't smile. Many wondered why he was so sour, so angry. Especially with Potter. Not James, but Harry. Harry who was exactly like his mother in many ways. Harry, who Severus hated so much and forced him to reveal those traits that made him like James. Harry, whose green eyes looked at Severus exactly like Lily had on that final day that he saw her alive- angry, hurt, and confused. Severus hated James' son, but he loved Lily's son. So he protected Harry whilst making the boy hate him.
Many people wondered why Severus Snape never truly smiled. The answer?
Love's to blame.