Humans have an oath, a promise they make to the world when they are born:
I will Survive.
Survive out of spite. Survive as a witness. survive as a warning to the future. survive so these stories do not disappear.
I will survive or else. -- Anon Guest
Every living creature greets their first taste of life with a complaint. Brief or lengthy, living is worth making a fuss over. Humans, especially, enter their lives and rage at it. Fighting from the first moments of their existence. They are weak, helpless, covered in blood, and raging against the universe for daring to challenge them like that.
Humans are great survivors. They start by battling their Deathworld from day one. They fight the weather, they fight to eat, they fight disease, disaster, and deprivation. They fight their worst and eternal enemy - other Humans.
Some grow up and continue on their Deathworld. Others go out and fight the Universe. They fight the enemies of civilisation. They fight civilisations that are their enemies. They just... fight. Some, judged the most battle-ready of Humans, fight only their own circumstances.