It wasn't that Humans were suicidal, nor are they intrinsically self absorbed.
They just understand better than even most Herd like species, that sometimes the only way to save the masses is to sacrifice the few, preferably themselves.
That doesn't mean they went quietly, or easily. -- Adam in Darwin
It only takes a moment. A split-second decision along the lines of, Here is where I stand and fight. The lucky few see it in the Human's eyes just moments before the Human plants their feet, turns their back, and tells the Last Lie. "You go ahead, I'll catch up," or, "I'll get the next one." A very rare few choose parting words like, "It's been an honour," or, "This was worth it."
In their not-so-distant barbaric era, Human tribes once favoured death in battle over death by old age or disease. They counted childbirth as an honourable death, since it was a battle to bring new life into the world. This does not mean that Humans ever took their own deaths lightly...
Humans go down fighting. To their last breath. To the last thumb on the last dead-man trigger. To the last spit of defiance in an enemy's eye. They make the enemy pay for every last inch they take, and, if possible, blow it up in the process. Humans not only employ Scorched Earth tactics, but embellish them to be certain that whoever decided invading was a good idea will never have that idea again. One way or another. Then there are the rare and mighty few who actually do make it through the fire and the flood and actually do catch up later. One such legendary Human is Andi the Undying.