Reynaldo must follow Polonius’s instructions because Polonius is his social superior, whom he must listen to and obey.
Detailed answer:
In Hamlet, these two figures have different social classes. Polonius is the advisor of Claudius, while Reynaldo is a servant. He must obey any instructions given by his superiors, including Polonius.
Polonius has no trust in his son, Laertes, who is going to Paris. Even though he encourages the young man, Polonius wishes to control him from Denmark. As Reynaldo’s social superior, he sends him to Paris to spy on Laertes:
…but breathe his faults so quaintly
That they may seem the taints of liberty,
The flash and outbreak of a fiery mind,
A savageness in unreclaimed blood,
Of general assault.
(Act 2, Scene 1)
Reynaldo has to gather information about Laertes’s whereabouts and reputation in Paris. Polonius instructs him to find any Danes who may know his son. Then, they could reveal useful information, reacting to Reynaldo’s terrible rumors about Laertes. Polonius recommends his servant to resort to lies if it helps the mission:
Your bait of falsehood takes this carp of truth:
And thus do we of wisdom and of reach,
With windlasses and with assays of bias,
By indirections find directions out:
So by my former lecture and advice,
Shall you my son.
(Act 2, Scene 1)
The relationship between Reynaldo and Polonius does not allow the first man to decline the order. The servant cannot refuse to become a devious spy in France. Reynaldo’s social class compels him to follow any instructions from his superior advisers. Therefore, Polonius’ evil plan to teach his son a lesson shows his true colors. It also indicates that he is suspicious of Laertes’ intentions.
Even before Polonius’s order to Reynaldo, he exercises and control over the servant. The advisor checks Reynaldo on his loyalty and understanding of what he does:
Do you mark this, Reynaldo?
[…]
You have me, have you not?
(Act 2, Scene 1)
Successful testing allowed Polonius to trust Reynaldo with such a dangerous task of spying on Laertes.