"My heart yearned to be known and loved by these amiable creatures; but I was an outcast from society; and my heart was filled with bitterness when I thought that I was the sole cause of the misery I endured. In the bitterness of my heart, I cursed the hour of my creation."
This quote is from Chapter 16 of the novel. The monster, feeling rejected and alone, expresses his desire for love and acceptance, and his deep sorrow at being an outcast. It's this raw, heartfelt expression of his suffering that finally breaks through Victor's guilt and causes him to weep for the creature he created.