The Mirror of Erised.
Severus turned the ancient door handle slowly. The door swung open on it's hinges noisy and he cringed hoping no one had heard him. Inside was exactly what he was looking for, the mirror that showed a person's deepest desires. Severus swallowed hard and walked towards it, cocking his head to examine it.
One too many students had stumbled across it's tantalizing beauty before and had told stories of what they had seen while peering into it's depths. Tales of wealth, fame and power were common amongst most students, but Severus knew what he deeply desired with all his heart.
It had been years since he first discovered it and decided it was best to live his life rather to get lost in it's mysteries. When he first stumbled across it he saw himself and slowly Lily Evan's walked up behind him and gave him a hug from behind. He could not feel it, but yet it felt so real. She turned to him, smiled and kissed him on the cheek and how he longed for his desire to come true, to have her for his own.
Though mistake after mistake, she drifted away from him and his dream of holding Lily Evans in his arms slipped through his fingers.
Now it was the night before Lily Evans', soon to be Potter's, wedding. Severus' body ached in pain as he knew by tomorrow her heart would belong to another's forever, his rival no less.
Severus stopped and stared into the mirror, awaiting to see his deepest desire. Severus' reflection disappeared and in his place stood a smiling Lily Evans. She wasn't with Severus, nor was she with Potter. She was just standing by herself smiling, happy and content.
Severus's throat went dry and he held back his tears as best as he could. He was going to march right on over to her house and confess his love for her. He was going to tell her how much he loved and cared for her; he didn't care how it would mess things up. He just wanted her to know how much he cared. That he had always cared.
Severus apparated to Lily Evans' house and stormed towards the front gates. Suddenly he stopped dead in his tracks as he starred up into the second story window. In front of the window was a smiling Lily Evans. She did not notice Severus, as she was too busy trying on her wedding dress. Alone in her room she laughed and twirled, giggling at the sight of herself.
She was happy.
Severus then realised. His deepest desire wasn't to be with Lily Evans. Just like the mirror showed him, all he really wanted was for Lily to be happy, no matter who she was with. It would be easy to waltz in there and confess everything but instead he turned and apparated away, leaving Godric's Hollow.
He wanted Lily Evans to be happy, and she was.