Thanks to a near-extinction event 75,000 years ago, all humans alive share an extremely high percentage of our genes with each other. This means that any two humans anywhere in the world are more similar genetically than two chimps from the same troop.
A reason for this is that humans might have almost gone extinct 75,000 years ago. The culprit is thought to be the Toba volcanic eruption that happened in Indonesia at around that time. The eruption was the equivalent of 1 billion tons of dynamite and it might have reduced the average global temperature by 5 degrees Celsius and started a new Ice Age.
This event might have left only around 1,000 to 10,000 breeding human pairs. From there, humans repopulated and prospered. However, the reason we share so much DNA is that we all come from those few thousands of people.