Why was my postmas so sarcastic this morning?

OK so my lovely postman today I discovered was s sarcastic dick. He was walking down my driveway and was about to post the letters through the letterbox,but I pulled open the door before he got and chance and asked "Have you got my letters?" And he replied by saying "no, I've got your milk". I mean I just asked a simple question and there was no need for the rudeness. He would usually skip to my door, whistling a tune.