He proposes that Hamlet's strange behavior is a result of Ophelia rejecting his advances. Polonius then uses this as an opportunity to spy on Hamlet, believing that he can manipulate the situation to his advantage. He suggests sending Ophelia to meet Hamlet, hoping to see their interaction and confirm his theory.
This is part of Polonius's overall strategy of manipulating and controlling both Ophelia and Hamlet. He doesn't genuinely care about their well-being, but rather sees them as pawns in his own game.