Polonius umes Hamlet has turned mad because?

The answer is "of unrequited love for his daughter." Ophelia tells him how Hamlet visited her and sighed and looked at her longingly. Of course, he is imitating madness, but Polonius thinks it is real love sickness. Or at least he probably thinks it is so. He also wants to believe it, because for Hamlet to marry Ophelia would be a huge step up in the family's fortunes (as he makes plain in his sycophantic speech to Claudius). Whether he deep down thinks it is true love is unknowable.