It is commonly cited that it was environmental pressures that pushed our forefathers to up sticks and venture out of Africa…
I like to think it was a deep-seated desire for exploration and adventure. After the advent of agriculture, boredom surely got to hold them and they could not explain the unsettled feeling that drove them to walk further and further from their comforts.  
Boredom is the opposite of happiness and these guys certainly know that we are at our happiest when seeking adventure.