Let's face it, Louis XV had a thing for his women, and his leisure time. Infamous parties, suppers, were held after a hunt, and for years these gatherings were a little hush hush. They were known of, but the guest weren't advertising their invitations.
Who hosted these gatherings? For a long time Madame du Barry was the acting hostess, and she was needless to say good at it in her own way. She brought to the table a different style than what was typical of court life. Her suppers were casual in manner, relaxed, and sexy. The atmosphere matched that of a "gambling saloon and the entertainments" were of the same sort.
Eventually these parties became the thing to do, and those who made it on the guest list were now walking around fanning themselves with their invitations- completely flaunting them rather than keeping it quiet. The suppers were private parties, about 20 guests would gather for them. One observer of a supper saw both men and women seated around a large round table in promiscuous positions.... take what you will from that! Madame du Barry sat at the kings arm picking food off his plate, flirting with him and whispering to him. This is a good representation of how our observer may have viewed it.
In any case, young Louis (the dauphin) was not typically on the guest list. He was way too innocent, and always under the watch of his tutor. Eventually, (it had to happen at some point!) with the encouragement of the Duc de St Mégrin, Louis attended his 'first' supper party!
Undoubtedly it was almost an initiation for the young dauphin, and his aunts, whom he was very close to, were not happy about it. They got to work on him right away, with stories of du Barry and her past. They painted a negative image of her and it really made a lasting impression on Louis. With complete trust in them he formed his permanent opinion on the favourite, which would never change much.
The Comte Mercy wrote of Louis "This enlightenment produced so strong an impression on the Dauphin that he has since evinced a marked aversion for Madame du Barry it is certain that she will never rehabilitate herself in the eyes of the young prince."