"The place where it all started... The place where I have experienced a lot of pain and suffering... I should be angry... I shouldn't be here in the first place... But I don't have enough hate to forget that you used to love me as your child..."

Zack is visiting his father's grave which is basically the place where all his pain and suffering began, which is his father's lab. Even after everything Zack had gone through, he still doesn't have enough hate for him to forget that his father used to love him. Before his father got consumed by their own research, he was a loving father to both Zack and his mother, and thanks to his father being close with Terrence (Rain's father), he wouldn't even have met Rain at all.