Seymour Glass is characterized by his peers as someone who is completely out of control; he was released from the military hospital, and no one seems to understand why he was released because of his ludicrous behavior. He does, however, seem to relate particularly well to a very young girl, Sybil.
Sybil's mother mistakes Sybil's comments of "Seymour Glass" with that of "see more glass," thus implying that Sybil is the only one who can see through Seymour as if he was glass. This becomes apparent in his story of the bananafish.
According to Seymour's story, the bananafish appear "perfectly normal" until they swim into a hole and overly consume large amounts of bananas in which they obtain "banana fever." This fever prevents the fish from being able to leave the hole, and thus they die from their excess. As Sybil is the only one who is able to "see more glass," she also has the ability to see a bananafish with "six bananas" in its mouth. The fish dies from excess of its own world; it entraps itself by overindulging, as Seymour does as well. Because Seymour relates perfectly well only with a child, his psychosexual development is nothing less than hairy, and his desire to kiss Sybil's feet, and hide his own body and feet, are much like the fish. The fish need to secretly swim into the hole to indulge in their fantasies, as Seymour seems to express his true feelings and desires away from acceptable society -- such as his awkward relationship with the young Sybil and the younger Sharon Lipschutz. Sybil leaves Seymour after denouncing him for kissing her feet, in a very child-like mild manner; he then proceeds to return to the materialistic world of room 507 where his wife Muriel, surrounded by the smell of her nailpolish, is asleep, and he kills himself.
Once the bananafish realize that they cannot escape the destruction they have brought upon themselves, or that they cannot return to "reality" or "society," they simply have no choice but to perish from existence.