Why does my boss treat me differently now that I've met his wife?

I temp as a secretary and in the beginning he was very nice to me and showed me photos of his trips away and just generally made me feel at home in the office. He asked me to cover until his new permanent secretary started called Rosie*. It later transpired that this Rosie* was his wife and it is her who'll be taking over from me. He pretended his new sec would be from outside. He never introduced me to her until one day another secretary recognised her and she introduced me to her as his wife, when I was working elsewhere in the company. Since I've met her, he has been quite stern, snappy and sharp with me and I don't know why. He is much less friendly with me now and is treating me coldly. Once I had lost a bag of sweets in the office and he was on the floor looking all over for them for me, but now it's as if I don't exist.