Sam came into Will's life just after Leah broke the news that she wasn't single. Immediately, Sam wanted to sleep with Will but when they first reached the bed, he was impotent. The same thing happened the second time they went to bed together. Sam was beginning to think it was her, but then saw it as a challenge. She succeeded in helping him get it up on her third try.
They started a relationship after that, but Sam hadn't realised it was a relationship, even though Will regarded it as one. This later caused trouble for her but was resolved reasonably quickly. Until Will started getting jealous of every guy she talked to, at least.
Towards the end of the series, after Will realised he loved her but before he got the chance to really tell her, she was killed in a car accident. Will was troubled for quite some time after this event.