Duncan thinks that Macbeth and Lady Macbeth are wonderful hosts and he is completely at ease. See Act 1.6. The opening lines find Duncan completely comfortable in both his physical surroundings and the company of his hosts. He tells Banquo:

This castle hat a pleasant seat. The air
Nimbly and sweetly recommends itself
Unto our gentle senses. (1.6.1-5)

Further, Duncan greets Lady Macbeth warmly calling her "our honoured hostess" and thanking her "for your trouble" (1.6.10,15)

Duncan is completely unprepared for the treachery that is to come. Assured that the king suspects nothing, Macbeth and his Lady further concoct their evil plot.