This incident occurs as Gatsby is leading Nick and Daisy on a tour of his home. Gatsby is absolutely overwhelmed with the wonder of having Daisy, the girl of his dreams who he has worshipped and idolized from afar for years, physically present and looking at his possessions. "After his embarrassment and his unreasoning joy he was consumed with wonder at her presence."
Gatsby's response to the situation is to share every piece of his existence with her. He escorts both of his visitors, but really only cares about Daisy, through every room, pointing out every wonderful detail or reflection of his success and refinement.
He opened for us two hulking patent cabinets which held his massed suits and dressing-gowns and ties, and his shirts, piled like bricks in stacks a dozen high.
Daisy, with her love of all things beautiful and luxurious, is first amazed and awed by the abundance of luxuriant fabrics and wonderful colors. Then she mentally compares Gatsby, the man who dresses in all these "beautiful shirts," with Tom, her husband the polo player, who dresses in riding clothes.
The mental comparison between the man she married and the man she dreams of being with may be the reason for her tears. She may be sad because she never has, and never will, see Tom in shirts like the ones Gatsby possesses.