As Slav says it happens when the other team lose interest, it also occurs sometimes when the transfer window closes, however, once the interest starts up once more he whines again. Had it happen with other players not just Pogba (De Gea gets grumpy when teams become interested as well and then when no offer appears he is wanting to return to the fold). You just have to stand firm and play chicken with PSG (oh and make sure you renew his contract when he is happy so you don't lose him on a freebie).