From a moment in Season 2 Episode 22: Becoming Part 2
Spike was carrying an unconscious Dru in his arms when he turned and saw the beautiful Buffy. And Angel, standing over her.
"My God, he's going to kill her," it dawned on him.
In that split second, he realized, he loved her. He was obsessed. He cared for her. Dru loved Angel, she would never love Spike, after all her did and felt for her. He dropped her unceremoniously on the floor and sprinted towards Buffy. It had felt good beating the Hell (no pun intended, as the gate to Hell would be there at any moment) out of Angel. He thought he knew why; Angel had taken Drusilla away from him. But no, it was because Angel had Buffy... And Spike wanted Buffy. Spike jumped on Angel from behind and started to hit him.
"Buffy, go! Close the portal!" Spike shouted, turning his head to check on her.
He then felt a tingle in his jaw, where Angel had swung a punch. Spike pinned Angelus down and hit his face over and over, left, right, left, right, over, over, and over again.
So, I just had this idea whilst watching Buffy, now you can finish it.