He thinks she'll never leave.  But she's drowning.  He's more than she can take.  Exhausted and broken, gasping for air, she turns away.  She's been up and down so many times, she doesn't know if she'll resurface if she goes under again.  Her survival instinct takes over.  She locks all the doors.  She ignores his cries.  She knows she must let him go in order to save herself.