As you can see in the video, the rat is chasing happiness. It bounces from one promise to the next, looking for status. As Clive Hamilton wrote in his book, Growth Fetish, “People buy things they don't need, with money they don't have, to impress people they don't like.”
However, it is important to know we never join this rat race by choice. When the majority of people follow a belief that says, 'in order to fit in, we have to appear successful', it's only natural that we too will adopt this belief. When our society reaffirms this belief 24/7, we believe it at such a deep level we assume it is who we are.
It's the same driving force that fuels all of our bad habits. Your subconscious, emotional mind (or what NYU Social Psychologist, Jonathan Haidt describes as your elephant) shapes your thoughts, feelings, and emotions towards satisfying that belief.