If you recall, the last time Capulet spoke to his daugher, it was with great anger and he pretty much told her to get out if she wasn't going to marry Paris, so when he speaks the lines:
Ready to go, but never to return.
O son! the night before thy wedding-day
Hath Death lain with thy wife. There she lies,
Flower as she was, deflowered by him.
Death is my son-in-law, Death is my heir;
My daughter he hath wedded: I will die,
And leave him all; life, living, all is Death's.
He is demonstrating great regret and compassion as now he refers to her again as his daughter which includes her in their family. He speaks with great emotion.
This passage also personifies death as the new suitor which has claimed Juliet as his wife. This passage demonstrates Juliet was important to him and now she is gone in the arms of Death.