Why does polonius say that he will spy on hamlets conversation with his mother?

Polonius claims that Gertrude will be too partial and forgiving to her own son.

Polonius says that he will spy on Hamlet's conversation with his mother because Hamlet has people who are wondering about the causes of his strange behavior and is determined to know the cause of Hamlet's bizarre and threatening behavior. Claudius and Gertrude retain Rosenkrantz and Guildenstern to spy on Hamlet, but they do not arrive at any viable reasons or anything useful on Hamlet's behavior. Additionally, Polonius and Claudius attempt to monitor Hamlet's and Ophelia's "private" conversation, where Polonius perceives that Hamlet behavior may be because he must be mad with his love for Ophelia, for she has distanced herself from him since Polonius ordered her to do so. Polonius speculates that this lovesickness may be the origin of Hamlet's moodiness, which he hurries out to tell Claudius of his idea.