Why is the young man compared to "his grandsire cut in alabaster"?

In the opening scene of the play, Antonio is depicted as a depressed, melancholy man and his friends Salario and Solanio attempt to discover the cause of his sad attitude. While both men guess that Antonio is worried about his merchant ships at sea or that unrequited love is the cause of his melancholy mood, Antonio insists that neither is the cause of his sad disposition. After Salario and Solanio leave the scene, Gratiano mentions that Antonio does not look healthy. When Antonio comments that he is simply sad for the sake of being sad, Gratiano comments that he would rather enjoy life by celebrating and drinking wine. He then asks Antonio

Why should a man whose blood is warm within Sit like his grandsire cut in alabaster, Sleep when he wakes, and creep into the jaundice By being peevish? (Shakespeare, 1.1.85-88)

Gratiano is essentially asking why Antonio or any young man would want to behave like an alabaster statue of a grandfather. Gratiano is challenging Antonio to have a positive attitude instead of moping around. Gratiano believes that young, healthy men should be active and celebrate their health rather than sit still like the statue of a grandfather (grandshire).