Buffy and Angel loved each other more than anything. "The relationship Buffy and Angel had was true love in its purest form."- SMG
I know everyone says Angel broods all the time, which is true, but the things he had done in his past caused him to distance himself from civilization. For example, the "whistler", when he met Angel in the Alley in Los Angeles, Angel told him to get away, afraid that his vampire instincts would kick in and he would want to drink him (since he was "all skin and bones"). When the "whistler" offered him a chance to become someone, he accepted and saw Buffy. He wasn't lusting after her! Anyways, he waited for her call to be the slayer and then he began his journey for redemption. Yes, he gave her cryptic comments then left, but they did help. He left because he didn't want her to discover him being a vampire. But, when she did, she tried to convince herself that he wasn't bad, which he wasn't. He refused to drink her and he didn't pull away when her cross (that he gave her) burned his chest. But, they couldn't stay away from each other. Their relationship began to grow. Before we know it, season 2, they begin to make out and stuff. (which wasnt a bad thing)

Now, the losing the soul thing. He lost his soul because he loved her so much. He achieved true/pure happiness in her, that's what cost him his soul, not the sex. He was happy with her and didn't want anything else in the world. I don't think anyone can fully understand the meaning of "true happiness". It's very deep. Anyways, He lost his soul also because of the gypsy curse. He is now Angelus. When Buffy needs to kill him, she tries to find another way by cursing him again, but, her and the scoobies fail. Now, when she does kill him, she grieves over him. She, in my opinion, broods. (tell me what you think in comments) But, she runs to LA and hides from her friends and family trying to run away from herself. Then she returns, but less spirited, as Parker said. Once she saw Angel, she didn't tell anybody because she was afraid they would want to kill him or hurt him, that they wouldn't understand, which indicates she still loves him. Everyone is more cautious around Angel (Giles, Amends, he has a crossbow) now that he is alive, not literally but back from hell. He has been brooding because of the things he had done in season 2. But, in Amends, Buffy directs him to the path of redemption, once again. She helped him. But, when she needed help, he always came through. Helpless, Earshot.
Let's begin with epiosde Helpless. She had no powers, and she felt "helpless". He encouraged her and helped her. But Giles did most of it. A moment of their undying love:
"Would you love me even if I wasn't the slayer?"
"I saw you before you became the slayer. I watched you and I saw you called. It was a bright afternoon out in front of your school. You walked down the stairs, and I loved you. I could see your heart. You held it before you for everyone to see and I worried that it would get bruised or torn, and more than anything in my life, I wanted to keep it safe... to warm it with my own."
This dialogue i think is important because he didn't love her not only because of who she is now, but who she was then. She was a regular school girl until she was called. He loved her before she became the slayer, and he always will.

In Earshot:
"You know I won't let anything happen to you unless I can help it. I'll love you, even if your covered with slime."
He doesn't care what she looks like. He loves her for who she is. He ripped out the heart of the demon to cure her. He ran in the sun for her, to give her the antidote.

When he left her, he did it for her, not because he wanted to try new things, not because he was selfish. Because he loved her so much, he gave up his happiness (not true happiness), his contentment, so she could have a regular life.
"You deserve something outside of demons and darkness. You deserve someone who can take you into the light. Someone who can make love to you."
He couldn't provide her with a regular life, her perfect life (what she said in "I Will Remember You"). So, he left.

In season 4, Angel returned to help her. To save her life, which he did. When she found out, she went to see him. He really is her guardian Angel. Of course, he can't be there for her all the time, but when he hears shes in danger, he would give his life for her, no doubt.

"I Will Remember You" definitly portrays their love for each other. When he was human, Buffy said, "This is always what I've wanted. A normal girl falling asleep in the arms of her normal boyfriend." He wanted her to have that by leaving. Then, he became human. Although, their happiness couldn't last because he was afraid she would fight and maybe die alone. He begged for her life and the Oracles took his life so she would live later on. This shows his true love for her.

*Please Comment, and try not to be too harsh, if you disagree, maybe we can meet half way, lol*