Why does the box of dog biscuits make Tom so emotional in great Gatsby ch9?

Because it made him to remember, and feel the loss and pain of Myrtles' death.

Explanation:

Tom describes his feelings to Nick to let him know that he has his own share of the loss. He was reminded of Myrtle, the wife of his mechanic, his symphathetic mistress who was killed by Daisy in a car accident. If Myrtle were to be alive, the box would have not been there.

Earlier before the incident, Tom and Gatsby had exchanged their cars. Daisy alone was driving the car when the accident occurred. But Gatsby tells Nick that Daisy killed Myrtle but he will take the blame.