It’s a law of life (which is based on the laws of chemistry and in turn physics). All creatures, including humans, seek to fulfill self-interest. Though technically, it’s the genetic information of the individual that is “seeking” self interest (there’s no conscious thought here of course, but in the competitive world, where resources for survival are limited, the ones that don’t seek self interest don’t survive while only self-interested genetic information survives). Still, for the most part, this means that an individual is generally seeking his or own her interest.
But wait; aren’t humans are known to be an altruistic species!? It is true that humans help out other humans on an unprecedented scale, but it’s under the guise of self-interest. An essential component to the uniqueness of humans is that their best self-interested strategy is to cooperate with and help out others. This doesn’t mean that we consciously or intentionally behave selfishly. Most of this behavior is hidden beneath our mind as subconscious processes.
One exception to self-interested behavior is when evolved mechanisms malfunction because they’re operating in a different environment than intended for, as is sometimes the case in our modern human world. Other than that, the rule of fulfilling self-interest will always remain. It scales beyond genetics, to other levels such as cultural information and especially entities within industries. While we sometimes find the behavior morally offensive, we should understand that it is completely rational.