Humans are interesting because they’re so active, but what this means might take a different form than some would expect. Dwarves are mining the depths of the earth and Elves are contemplating ancient mysteries; but Humans are building empires, settling new lands, and fighting titanic wars with one another.
This cultural variety is why most campaigns have multiple human nations, each with its own culture. That’s the way of humans—to divide themselves up into different societies with different behaviors, customs, and aspirations. Elves, on the other hand, tend to have a single culture in most campaigns, even if their communities are spread across the campaign map and they’re not politically affiliated with each other. Put simply, elves usually act like elves. But it doesn’t mean much to say that humans act like humans.
Energy & Ambition
Humans are supposedly tenacious when they set their minds to something, not as stubbornly as dwarves nor as farsighted in their quests as Elves.
Society's Drive to Organize
Human societies thrive on organization, perhaps because it’s the only way to keep such a varied people under control. A halfling or a gnome might enjoy cataloging a personal collection for the personal satisfaction that organization provides. But human societies regard organization as a means to an end: a more productive, efficient community.
Take city streets as an example. In human cities, the streets are straight whenever possible, intersecting with others to form a grid pattern. Elves regard a grid layout as unimaginative, and their communities often have winding roads that encourage contemplation of the natural world. The underground cities of the Dwarves have a regular network of tunnels them, because the dwarves find regular patterns to be inherently pleasing.
Humans save their creativity for other aspects of their lives, however, and they don’t necessarily like a street grid for aesthetic reasons. They like a straight grid of roads because the grid makes travel across the city faster and because it’s easier to assign addresses and give directions if the roads follow a predictable pattern. In other words, the organization is just a means to the end of societal efficiency and “progress.” Most tellingly, when the landscape does not allow for straight roads, humanity forces the landscape into shape, bending the world to the society’s will. Others call this arrogance, but humans call it progress.
Aggression
Human ambition is usually tied to another human trait: aggression. Humans have a primal desire to take what they do not possess and to expand their reach as far as possible. This aggression often translates to violence. As a result of this aggression, other races are often wary when dealing with humans. Nonhuman diplomats everywhere know the aggressive human mentality and are mindful that humans may push too hard or immediately make war to get what they want.
Focus on the Present or Future
Their short lifespans contribute to them spending less time looking back and remembering the past, and more time with their present or their future. A Dwarf may undertake an adventure to prove himself worthy to his ancestors (past focus). A Halfling might adventure so she can revel in the excitement of the moment (present focus). But a human will adventure to earn a knighthood and thus a better life for himself and his children (future focus).
Humans also live in more turbulent societies than other races. Human empires are always emerging and collapsing; even a well-established human nation might be younger than an individual dwarf or elf. The rise and fall of nations means that every human generation is touched by a revolution, war, or other calamity. Humans thus take less for granted; they assume that the world will change under their feet, and they employ their ambition, adaptability, and aggression to control (or at least survive) the changes.
they are driven by spite greed and ambition to try to change the world around them. the problem is they tend to succeed without thinking or seeing the consequences for themselves. They either don't live long enough to see the ramifications of their legacy nor KNOW exactly what their legacy will do up to a point and push for it. They do things just to make a name without knowing or caring about the consequences, with little known which of the two is a lesser of two evils.