However, Buffy still hasn't realised that she needs the darkness( which Spike can give her) and Spike still hasn't understood that he needs the light (which she carries) in order to be a better man. They need each other. Ying and Yang. But Buffy's been torned out of heaven and suffers. She hasn't got time for life dilemmas. They only bring pain and dissatisfaction in her life. The fact that she doesn't belong anywhere in this world crushes her.
Spike who once looked up to Buffy as a hero, who tried to change for her, now sees this fallen angel who can't cope with the situation, who can't find her place on this earth. He decides to step in and she decides to search and find her peace with him. She can't deal with this hard life right now so her only other option is death. We all know that it's a lot easier to be bad and to give up than to be good and to try. So instead of helping each other, Buffy and Spike hold each other back. Buffy doesn't have the strength to give Spike her light and she gives in his darkness. He accepts that and he lures her in it. They have sex which results in badness.
 

 

Buffy still can't accept her dark side and i think this lasts till season 7. She acknowledges the fact that it's there but she can't fully take it in as part of her.

Nothing's changed really. She still wants to be the shiny sunny Buffy she was but she can't understand that the clouds were always there.

Then the AR scene happens and we never get a chance to explore the dark side of the Spike/Buffy relationship again. Season 7 is a whole new beginning for Spuffy. It's about the bright side of the 'ship. The darkness seems to be gone.
 

But it's still there. In Spike.
 

Spike can't fully accept the light just like Buffy can't accept the darkness. By the end, though they finally realise that. Although Buffy knows it all along.

It would take some time before she finally admits it in season 8:

"You're my dark place, Spike."

Spike becomes aware of this as well:

"It was the best night of my life."

In the end they both find what they're looking for. Buffy--the violence which is part of her and which she has to accept and Spike--the tenderness and true love. They finally become one ♥.