Although they didn't act like it, they were made for each other. They were married, of course. They even had a son. But they couldn't show how they really felt for each other.
The Malfoy family was a proud, pureblood family. They hid themselves behind perfect Slytherin masks. If they ever let those masks fell in bad company, they'd probably die. Every person they met agreed they were typically Slytherins. They were cunning and ambitious. They were powerful and clever. But behind their unbreakable masks, they were different.
Narcissa Malfoy was a strong woman. She was perfect for him. Born of a pureblood family, sorted into the noble house of Salazar Slytherin, proud, devoted, a good duellist. She was the one.
Lucius Malfoy was a mystery. He never showed his true self to anyone, except her. She could appreciate that. When he proposed, she said yes. How could she ever say anything else. She did actually feel something for him, but couldn't really name what. Later, she found out what it was exactly that she felt. She did really love him. And she would find ways to show him. A few potions after an extremely exhausting death eater revel. An extra nice diner on special days. Doing extraordinaire efforts to keep their only son safe. Supporting him, whatever he decides. She would always stand by him.
He knew she loved him. He could tell. The way she looked at him. The way she talked to him. The way she would hold his hand. He also knew he loved her. He loved her more than himself. He couldn't show it, but he would find ways to let her know. He would always respect her and her family. He would support her in important decisions. He would ask her opinion on important things. He would let his son attend Hogwarts because she insisted he would.
They knew they loved each other. They knew their love would never die.