Disclaimer: Don't own Buffy or Angel. Yadda yadda yadda.
Faith would always love Angel. He believed in her when everybody else had given up hope, written her off. He understood her in a way that no one else ever could, because they were the only two people to experience what they experienced. Nobody else could ever have any idea what it was like to be out of control, to be evil, to harm people because you can, because you enjoy it, and then to feel remorse. To come back down. Sure, there were some people out there who killed for fun, and felt bad about it, but it wasn't the same when you had preternatural strength. When you were designed to kill, you could kill better. No one else alive, well, walking the earth, had any idea what that was like.
So, she loved him. How could she not? All the times he tried to talk to her, all the effort he put into making her better, helping her. She fell a little more in love with him every time he tried. She couldn't kill him, not even to make the feelings go away, not even to gain her freedom.
She knew he would never love her back. She had known that from the start, though she hadn't accepted it until he had helped her out of her darkness. He would never be in love with her, but he would always love her, in some way. And she wouldn't deserve even the small place in his heart he held for her unless she truly made amends. So she turned herself in. And she would do everything in her power to make things right with Buffy, the woman he was in love with, and the only other person who had ever tried to help her, ever could possibly understand her.
Buffy would always love Angel. He came to her in the night, of course, because he couldn't came in the day, and immediately enchanted her. She pretended that he hadn't, that she wanted nothing to do with him, nothing to do with any of it, but she wanted nothing more than him. When she found out he was a vampire, it only increased her desire for him. He was everything he couldn't be, evil and good, a vampire in love with a slayer. A slayer in love with a vampire. One of a kind, the only halfway point between vampire and human.
She loved him because he was dark, and exciting. She loved him because he was forbidden. But mainly, she loved him because he was Angel, because he fought so hard against his dark destiny, because he would always be there for her when she needed him. Because he wanted so much to make a difference, to be good. Because he was the only one she could trust to fight by her side, the only one she could trust with her heart.
She knew they could never be together. If they stayed together, no matter how hard they tried to prevent it, he would inevitably end up losing his soul, and going on a killing spree. Then, there were all the reasons he had left her. How could he bear watching her grow old, and die? And that was assuming that she even had a chance to grow old. She couldn't bear letting him watch her die.
Angel would always love Buffy. The first time he saw her, lost, confused, and dazed, he fell in love. He watched in the shadows as she learned about her calling, and he knew he needed to help her. He knew that someone needed him, someone he wanted desperately to help. He stopped living off rats. He did what Whistler wanted him to do, he began to search for redemption, to be worthy of the beautiful, powerful, good-hearted blonde who haunted his dreams.
The better he got to know her, the more in love with her he fell. She was brave, and pure, and utterly irresistible to him. He was constantly running away from her, because he was afraid to touch her purity, the purity that was sacred to the world, and to him. He was scared to touch her, to love her, for fear of pulling her into the shadows with him. He had tortured a lot of people in his day, but the memories that weighed the heaviest on his conscience were the memories of hurting her in any way.
He left her out of fear. He loved her too much to let her spend one second in the shadows that she didn't have to. He wanted nothing more than to hold her in his arms for the rest of his eternal life. But not at her expense, and that was the only way he could have her, so he let her go. He was afraid to be near her, not only because it meant he was holding her in the dark, but because every time he held her, he felt closer to perfect happiness than it was at all safe for him to be. She was perfect happiness to him. She was everything to him.