Leah was the love of Will's life. Yet in the series, she caused him more pain and hurt than she did him any good.
When Leah came back into Will's life after he'd started his relationship with Sam, she put a strain on Sam and Will's relationship. She unfortunately still had a strong hold on Will and wasn't quite willing to let go.
She of course, eventually admitted that she knew what had been going on and apologised for everything that she had done.