Spike could not deny his jealousy.
Buffy could have any man she wanted. She had the looks, the attitude, the capability to be wanted. But she did not want him.
Sure, Buffy would come to him almost every night, and they would do naughty things in his crypt. But she did not truly want him. No matter how many times Spike tried to make her realize that she came to him because she wanted him, he knew it wasn't true. Buffy wanted Angel, or Riley. Spike was there for her to play with until she got bored, and she would then kick him in the head and leave.
Spike knew she was looking for a mate. She refused to look in his direction. Many times Spike had gotten stone cold drunk and broke things trying not to think of her with another man. He was in love with her. Why couldn't she accept that?

~~~~~
Buffy could not deny her jealousy.
Spike could have any woman he wanted. He was more than good looking, and his attitude screamed 'bad ass.' He wanted her; she knew that much. But she was almost sure he had other women.
Spike had brought a date to Xander and Anya's wedding. A date! Jealousy had flared inside Buffy like flames in a forest fire. Had he kissed her? Had he slept with her? Buffy had every reason to be jealous, of him, and his date.

~~~~~
They could not deny their jealousy.
When Spike arrived at Buffy's house to find her in tears, he pushed the feeling aside.
"What's wrong, pet?" Spike asked her, sitting next to her on the front steps.
"Do you love me?" Buffy asked. Spike blinked in shock.
"Always, luv," Spike responded.
They kissed. A long, slow, intense kiss. Buffy would never admit that she felt for him. All she could do was enjoy his kisses, and not think about the jealousy that threatened to overwhelm her each time she thought about him sharing a kiss with someone else.