Each and every Guardsman is nothing more but a weak sack of flesh, blood and bones, uncared for by anyone. They face genetically engineered psychotic killers in power armour that also make use of literal magic, while being on drugs. They face deamons that seem to be ripped out of Hieronymus Bosch painting, squared when it comes to looks, cubed when it comes to deadliness. They face murderous space elves that are better than them in every possible way to the point when their farts smell like roses and their shit is diamonds, yet are likely to torture whoever they capture for literal eternity. They face sentient mushrooms that refuse to die and consider turning people into fine red mist their most favourite pastime. They face ancient skeleton robots that are literally next to impossible to kill with technology that doesn't comply with laws of physics. They face the most vicious zombies that ever happened upon science fiction. They face swarms of lizard-insects that are as countless as stars and the number of sharp appendages they have is twice that. They face magically induced depression, countless ilnesses that melt their flesh off, their shitty equipment and equally shitty luck.
They came from nothing, are nothing and will be nothing, as noone will ever note what they did!
And yet, for thousands of years, Imperial Guard stands fast! For thousand of years, Black Crusades have been defeated! Deamon magic has been defied and proven to not protect against ammased artillery fire! For thousand of years space elves continue to be a dying race thanks to valiant effort of humans! For thousand of years sentient mushrooms are stopped from overtaking the Galaxy! For thousand of years the impossible to kill skeleton robots are being killed! For thousand of years space zombies are keept irrelevant! For thousand of years the swarm is denied the juicy shiny flashlight of Terra! For thousand of years the Guards laugh in the face of all odds, keep fighting despite their deaths being long overdue, use their glorified laser pointers to bring down literal Gods and prove that Lady Luck is their bitch!
FOR THEY COME FROM TERRA, ARE IMPERIAL GUARDSMEN, THEY WILL HOLD THE LINE, AND THEY SHALL BE FOREVER REMEMBERED AS THE EMPERORS FINEST!
TL;DR
Humanity Fuck Yeah.
Also Death Korps.