Act 1, Sc. 5, lines 85-89: Why might it be hard for Hamlet to obey his father's wish that he not punish his mother?

Ghost: But, howsomever thou pursues this act,

Taint not thy mind, nor let thy soul contrive

Against thy mother aught. Leave her to heaven

And to those thorns that in her bosom lodge

To prick and sting her. Fare thee well at once.