Before Joel Quinn, Claire Sullivan really cared for Serena Sullivan.
She was willing to do anything to give birth to Serena, which showed how much she cared.
As Serena grew up, increasingly resembling Mr. Sterling, coupled with the instigations from Joel Quinn and his daughter, Claire unconsciously began to resent Serena, thinking Serena couldn't stand her happiness.
That hatred wasn't entirely directed at Serena; subconsciously, she transferred her feelings towards Mr. Sterling onto Serena.
Seeing through the true nature of Joel Quinn and his daughter, Claire could no longer remain deluded.
Her love for Serena erupted completely, no longer continuing to hate her because of Mr. Sterling.
When Joel Quinn wanted to harm Serena, Claire couldn't accept it and was only thinking about protecting Serena with her own life.
She hoped this way she could fulfill her responsibility as a mother and, in doing so, earn some forgiveness from Serena.
