Answer:
Polonious wants to prove that he is loyal and honorable. He wants to be the one that figures out whay Hamlet went crazy. Therefore, he wants to win the approval of the King and Queen and earn their respect.
Step-by-step explanation
Polonious wants to show that he is loyal to the Crown because he told her daughter to dismiss Hamlet's love proposal.
Besides, he wants to be the one that answers the big question most characters have "Why did Hamlet go crazy?" Polonious believes that it was because of Ophelia's rejection.
Polonious tells Gertrude and Claudius that he found out about Hamlet being in love with Ophelia, so he told her to dismiss him and any messages he could try to send to her. This was so, because Hamlet was a prince and Ophelia just the daughter of a noble man (“Lord Hamlet is a prince out of thy star. This must not be.”Act 2 Scene 2 Line 132= Hamlet is out of your league)
Polonious by telling her not to accept Hamlet (which was a very advantageous situation for the family) wants to prove how loyal he is to the Crown. He shows that he is apparently loyal because he told her daughter to dismiss Hamlet's love proposal.
Bear in mind that Polonious is the one character that provides comic relief during this tragedy, so most of the things he does are rather ridiculous, and he is always concerned about appearences and he overprotects his daughter Ophelia and his son Laertes. In fact, the real reason why he tells Ophelia to reject Hamlet is because he wants to protect her and he doesn't approve of their relationship.
In Act I Scene III Line 127 Polonious warns Ophelia, but in its context, we see that he is a concerned father, protecting her because he believes that Hamlet's intentions are not "holy".
"Do not believe his vows, for they are brokers
Not of that dye which their investments show,
But mere implorators of unholy suits," ( = don't believe his promises, he shows something that is not, he is only trying to make you fall for him and to make you do "unholy actions".)