Why was David's mother nice? Why did he believe in her?

David's mother changed her tactics because she was expecting a visit from child services. David believed in her because he wanted to.... because he desperately needed to feel loved.

Before answering, I looked up at Mother, who smiled politely. I felt as though a bomb had exploded deep in the pit of my stomach. I thought I would throw up. It had suddenly occurred to me why Mother had changed the day before; why she had been so nice to me. I felt like a fool because I had fallen for it. I was so hungry for love that I had swallowed the whole charade.