why do ya think Draco cry in Harry potter and the half blooded prince?

Are you talking about when he was in the bathroom? In the book it said why: His mission was to kill Dumbledore and, up until that point, he'd failed every time to do so. Voldemort had threatened to kill him, his mother (and, presumably his father too) and he was afraid that, since he'd failed so many times, Voldemort would kill them all anyway.

Draco has been given a job (not that he had much choice...) for which he is not cut for. He's undergoing a tremendous amount of stress as he sees more and more that he is not up to the job he had been given by Voldemort : killing a man. Not any man : killing a far more powerful wizard than him. He's downright scared of both Dumbledore and Voldemort. He is aware that if he fails, he and his family will pay the high price.

Like his mother said, he's just a boy. Any boy or girl aged 16 would be as much distressed as he was then. I feel sorry for him.

well he was assigned to kill Dumbledore and thats hard for him to do,considering he was so young and had never killed before,and he thought Voldemort would kill him and his family if he didnt and im pretty sure that can make anyone cry. :P

i think b/c lord master was terrorizing him

He felt remorse for what he did to katie.

Draco was very confused and sad at that point. He had failed Voldemort many times. Voldemort had accused him of being, basically, worthless. Draco had tried and tried to make Voldemort see that he was worthy to stay a Death Eater. As for the confused part, he didn't know what to do to make himself worthy again.

I feel so sorry for Draco as I know exactly what he's going through. I have tried and tried to make myself worthy to someone for years, but I always fail.

God bless Draco for not giving up.

Por que ele não consegue matar Alvo Dumbledore, mesmo depois de inúmeras tentativas, como a garrafa de hidromel envenenada, o colar amaldiçoado, e Voldemort ameaçou matar a sua família e ele, se ele não conseguisse.