What does "like a starving man looking through a window at a banquet" mean ?

This quote from Natalie Babbitt's well-loved novel Tuck Everlasting is a literary device known as a simile: an expression that uses the word "like" or "as" to make a comparison. The phrase "it was as if he were entranced and—yes, envious—like a starving man looking through a window at a banquet" describes a character (Tuck) whose facial expression while he is looking at a stranger is being compared to that of someone who is starving and envious of someone who an array of foods laid out before him. The use of a state of physical need and pain, connected directly to human survival, to describe Tuck's state of mind underscores the depth of his emotional need and envy.
Tuck is immortal, and the sight he is gazing upon is that of a man who has just been killed. He is fascinated and also wonders about this state of death, of not being alive anymore, since it is something that is denied to him. The fact that it is compared to a banquet seen by a starving man is an intriguing inversion of what we would normally associate with starvation (a potential cause of death). To mortal humans, Tuck's ongoing existence would seem like the banquet, full of infinite possibility; but to him, he is a starving man hungry for the full experience of being human, including that final event of death.