From the start, Nick senses an enigmatic quality to the relationship between Daisy and Tom. The first night he comes to their house for dinner, he learns that Tom is having an affair. Daisy professes to a jaded unhappiness over this and insists she has gone everywhere, done everything, seen everything, and is, she states scornfully, "sophisticated."
Nick notes in response that:
I felt the basic insincerity of what she had said. It made me uneasy, as though the whole evening had been a trick of some sort to exact a contributory emotion from me. I waited, and sure enough, in a moment she looked at me with an absolute smirk on her lovely face as if she had asserted her membership in a rather distinguished secret society to which she and Tom belonged.
He also wonders as he leaves why Daisy stays with Tom:
It seemed to me that the thing for Daisy to do was to rush out of the house, child in arms—but apparently there were no such intentions in her head.
All of this foreshadows what will happen in the confrontation between Tom and Gatsby over Daisy later in the novel: Daisy will choose Tom despite his shortcomings.
As Nick intuits from the start, Daisy may be a drama queen who enacts the part of the injured, victimized wife of an adulterous man, but she and Tom have a bond that transcends that role—they share in the "rather distinguished secret society" that unites them. Nick realizes, too, that although she has every reason to leave Tom, Daisy has no intention of doing so. Something deep connects them, a bond that has developed since Gatsby faded from the scene.
As readers, we are never sure exactly what it is that holds them together, but the implication is that it involves Tom's high class status and vast wealth that allows them to live as if they are superior to the rest of the world. Daisy may toy with Gatsby, as Tom does with Muriel, and Daisy may even have a deep affection for Gatsby based on once having fallen in love with him, but none of that matters against the deep bond that she and Tom share.
Tom and Daisy need and complement each other. Daisy needs Toms relentless arrogance, strength, and money to feel safe, while Tom relies on Daisy's beauty, dependence, and charm. They will always stand by each other.