How are Daisy and Tom careless?

Daisy and Tom are careless because they affect the lives of the other characters without considering the consequences of their actions. For example, Tom engages in an affair with Myrtle, a comparatively lower-class woman, and he allows Myrtle to believe that he would, at some point, leave Daisy for her. In reality, however, Tom never has this intention. We know this because Tom competes with Gatsby later on in the novel, a fact indicating his devotion to Daisy. Tom's careless stringing along of Myrtle ultimately results in Myrtle's death when she tries to escape her husband, Wilson, with the erroneous belief that Tom would take care of her. Similarly, Daisy is careless because she engages in an affair with Gatsby without fully committing to him. Even though Daisy initially gives the impression that she would, in fact, leave Tom for Gatsby, she ultimately chooses Tom, leaving Gatsby to answer for the death of Myrtle even though Daisy was the one driving the car that killed her. Her willingness to allow Gatsby to take the blame for the accident shows the disregard she holds for the consequences of her actions. 

On a more basic level, Tom and Daisy both live a literally carefree lifestyle. They both come from rich families and spend their days in idle relaxation. They don't have to worry about money or jobs, and even though they do have a child, neither of them are shown to be particularly conscientious parents.