Upon discovering that Juliet is dead, Lord Capulet has a lot to say about himself and his grief; but he never wonders why his daughter died. He simply cares about how her death affects him. Nevertheless, he says the following:
"O child, O child, my soul and not my child!
Dead art thou, alack, my child is dead,
And with my child my joys are buried" (IV.iv.89-91).
What Lord Capulet means in the above quote is that all of his joy was placed in Juliet; therefore, with her death, it is as if all of his joys are also gone forever. Another way to say it is he placed all of his future joy in Juliet marrying Paris, providing him with grandchildren, and doing everything that he wanted out of this time in his life. Earlier, when Paris first comes in to find Juliet dead, Capulet says to him that "Death is my son-in-law, death is my heir" (IV.iv.65). This proves that he was hoping for grandchildren, probably a male, to give his wealth to after he dies. Now, that can't happen. There are no heirs to the Capulet estate and this is his greatest regret.