What remarks by Lord Henry upset Dorian?

In Chapter 15, Dorian attends a party thrown by Lady Narborough, a wealthy widow and popular socialite. He acts naturally and comfortably, charming his hostess and successfully masking his tortured conscience, but is unable to stomach any food. Most of the guests are dull and witless, so Dorian is glad when Lord Henry arrives. As usual, Dorian delights in Henry's paradoxical, slightly offensive witticisms. The evening goes smoothly until Dorian is asked how he spent the previous night. He founders and retracts several answers, clearly discombobulated and unnecessarily defensive. Henry can easily see that something is wrong, but when he tries to get Dorian to share his troubles, the younger man excuses himself, saying that he is "out of temper" and "must go home."