She couldn't tell her.
Bellatrix already hated him enough; the truth could not come out. It was a wonder Bellatrix didn't ask Narcissa how she knew where he lived. Narcissa had expertly maneuvered through the streets and houses, mainly because she had been to his house many times before. Oh, it was so wrong. But Narcissa couldn't help it. She loved him. She loved Severus Snape. But she loved her husband Lucius, also. Loving two men at the same time was hard for her, keeping it a secret even worse. She feared Draco or her husband would find out, or even worse, the Dark Lord. After Narcissa and Bellatrix left Spinner's End, Bellatrix had questioned her sister fiercely.
"Cissy, is there something going on between you and Severus!?" Narcissa glared at her sister, easily masking her guilt.
"Bella, how dare you accuse me of cheating on my husband!? I love him! Do you think I am so low?" Bellatrix balked and shook her head.
"No Cissy, it's just that it seemed like he agreed too easily, is all." Narcissa glared at her sister who was silent the rest of the trip to the apparition point.
"I will see you later Bella, please be careful." Bellatrix nodded her head and apparated away, leaving Narcissa alone at the point, afraid to go home to an empty house. With Lucius in jail and Draco visiting friends and training to be a Death Eater, she saw little of her boys. That loneliness drove her to Severus time and time again. Only he could fill that void in her heart right now. So she walked back to his house, quickly, not sparing a glance over her shoulder for fear of what she might see.
How could I make an Unbreakable Vow? Severus thought to himself, large cup of wine sloshing around in his hand. It was simple: he loved her. He had taken the Unbreakable Vow because he loved her. He cared for Draco, and did not want him to be harmed, but he loved Narcissa. He would do anything for her, and had made the ultimate sacrifice. For her. But an Unbreakable Vow? The self-centered part of his brain kept repeating. He was afraid of telling Narcissa the truth. That he was a spy within a spy. The Dark Lord thought he was spying for him against Dumbledore, but it really was the other way around. But how could he tell her that? Maybe she already knew; she had been giving him interested looks when he had explained everything to Bellatrix. But if she didn't, then he couldn't tell her anyways. Her husband worshipped the Dark Lord, and as his wife, she must also. How could he tell her he loved her? She had a family, and he just couldn't compete with that in the end. It just isn't fair... He told himself that again and again, worried because their long affair had to be revealed soon, it was inevitable. He was worried to tell her the truth about everything, and that he loved her, never wanted to let her go. Stay with her always, be by her side, and grow old together. The problems of Lucuis, Draco, the Dark Lord, and Dumbledore were too great though, and they were doomed to be together, and separate, always.
He took a large sip of the wine, for once not caring if he had no sense of decorum, as the wine dripped sloppily down his sharpened face, making it appear much harsher and scarier than it could be. He slipped down onto the sofa, morbid thoughts running through his head. Doomed if I do, doomed if I don't... ran through his head, echoing off the silence of his lonely house and shattering his sanity into a thousand bits. He slowly and painfully fell asleep.
She finally made it to his doorstep, and raised her delicate porcelain hand to the unkind cherry wood, and stopped. I hate myself for loving you, yet I love you just the same. Then why can I not tell you? Have I shown you my love, can you guess? Or have I been the cruelest of all? Did I lead you into a situation that will end up killing you? Narcissa then sunk onto the ground and wrapped her arms securely around her chest, protecting her heart. Why does it hurt so much? Why? She cried for some time, before picking herself up, wiping away at her face, and walking through his door with the key he had given her. When he saw her, woken from his troubled slumber, she appeared like a perfect porcelain doll. He could not detect the pain she had been through on his doorstep. But if he tried hard enough, maybe he could see the cracks starting to form on her, worn and old with age. All beautiful things are harried sooner or later.
He tried to get up to her, but nausea stopped him. Instead, she came to him, once again performing their relationship out for them to see. She sought him, and he, emotionally unavailable on the outside yet hopelessly in love on the inside, did not know how to go to her, so she took charge. She sat down on the couch next to him and laid her sunlit head in his lap. He gently stroked the blonde waves, relishing the silkiness of each fiber. She sighed and engrained this feeling of strange love into her brain; the good, and the bad, the exuberant, and the depressing, bursting like fireworks all at once in her head and heart.
"Severus, I never told you I love you." He smiled but inside choked up. He hadn't either.
"Narcissa, I never told you that I too, love you." She smiled, but inside choked up. It couldn't last like this forever. Change was coming. But she was content to stay here, leaning on him, until it was time for her to go to her lonely home, and once again, become part of a different love. It was their destiny.