Well, humour may be healthy for the person doing the chuckling, but isn't always appreciated by others. As Delene says, he must be feling awfully grumpy --- imagine if he had just received bad news at home, or news of some awful problem within the company --- then the laughter might have been especially galling for him to overhear.
In this case, from what you say, maybe he found he was more behind than usual, plus having the flu, and recognizing that you were a happy bunch and he is not one of the club.