Why does Xiangzi feel uncomfortable looking at the backs of his grandfather and grandson?

Because Xiangzi is very kind and sympathizes with the two men who are about to starve to death.

The character Xiangzi comes from Lao She's "Camel Xiangzi".

After Xiangzi’s car was taken away by the soldiers, he always wanted to have a car of his own. When Xiangzi was waiting for Mr. Cao who was going to watch a night movie in the teahouse, he met an old man and a young man pulling a cart (the old man pulling the cart and his grandson Xiaoma). He saw the old man’s extremely broken-down car. Car, but this dishonorable foreign car carries all the lives of the grandfather and grandson. Watching the two people leaving, Xiangzi felt an unprecedented sadness in his heart. Their tragic situation shattered Xiangzi's greatest hope. Not only was the little grandson his past, but the old man seemed to be It is your own future.

"Camel Xiangzi" is a novel written by the people's artist Lao She. It describes the tragic fate of rickshaw pullers during the warlord war in the 1920s. Xiangzi is a representative figure of the working people in the old society.