People often misunderstand what makes humans so dangerous, they look at their weapons and say that is what makes them dangerous but every species has instruments of war of devastating power. They look at their ships and say that is what makes then dangerous but there are many species with much more advanced ships then humans who fear the humans just as much as everyone else does. They look at humans disposition and say it's their warlike temperament that makes them dangerous but we have dealt with many war races and none have survived long enough to be a real threat.

What makes humans dangerous is based around what most consider one of humans most glaring weakness, their lifespan. For the average species the 3 most common causes of death are; accident, homicide, and finally suicide. Most species are resilient enough against disease that it's a major news story when anyone in the galaxy dies from one and to the best of our knowledge humans are the only species without the gene that prevents telomere decay past maturity and is the cause of what humans call ageing.

For those unfamiliar with that term, due to the telomere continuing to break down it causes errors in transcription of DNA which manifests as muscles loosing strength and elasticity, weakened immune responses, slowed and limited ability to heal damage to the body, decreased mental capacity in both recollection and intake, and a unique category of disease called cancer where the error in replication that runs rampant and eventually kills the human. That last one is inevitable in all humans even if death through all other means is avoided through medical care due to how their DNA damages over time just by the act of cellular division.

You may wonder how something that only brings negatives and ultimately limits human to lifespans in the hundreds of years with the last 1/3 being in a state of extreme infirmity is a strength. The answer is simple birthrate, most species will have 1-2 children per millenia when they are actively trying to have kids. Humans can have a child and start incubating the next one in a single year and while they normally carry 1 child at a time they are capable of carrying multiple (2-3 seem to be the normal deviation but we have found records claiming up to 10 with medical assistance). Their gestation and development are also extremely fast. For most species the gestational period is around 10 years and developmental stage complete between 150-200 years; humans on the other hand have a gestational period of 9 months and developmental period of 16-20 years.

This rapid breeding is why all attempts to cull the human population have failed. War is rare in galactic civilizations because it's too easy to accidentally kill to many people for a species to recover when you can only add half your population again per 1000 years, a single planet wiped out is a major blow to any civilization but humans can recover from that in a fraction of the time it takes anyone else to. Humans can afford to fight wars of attrition without worrying about repopulation and they know that so they turn every war into a war of attrition. They will gladly take a 1000 to 1 losses in a battle knowing that they will have those deaths back within the year and the new soldiers fully trained and outfitted before their enemies have started to replenish their troops.