Arthur and Mordred confession

HPMad I can't believe Arthur hasn't clicked on yet that the people he puts above all others (excluding Merlin,Gwen and Gaius) always betray him (i.e Morgana and Agravaine) .You thought he would be a bit more cautious on the friends he choose's ,you thought he would have learnt by now. Why doesn't he check into peoples backgrounds before accepting them in his kingdom.One day he will learn.