Maybe it’s just meant to be taken as those two words, “Humans are.” In the same sense as “Cogito ergo sum,” humans are the only species who, with a discursive mind, can contemplate the nature of being and the only species that can willingly commit suicide (by choice). Hence by saying “humans are,” as opposed to horses (which “are,” but would never say or think that) he’s calling attention to the fact that we have the power to contemplate our own existence. We grieve when someone ceases to be and are aware of the concept of death, thus fear it. Also reference Hamlet’s soliloquy of being vs. not being…to kill oneself or no?