Selfishness is a simple knee-jerk choice of the moment.
It is based on self-preservation, greed, fear, or simple hungry need -- for the most part, id-based impulse.
Whatever its basis, it is a short-term approach to things. Acting selfishly might benefit you now, but not so much in the long term.
Take an extra moment before making your self-based choice to visualize ahead to the greater chain of events that will ultimately affect you and those that matter to you. Look days, months, years into the future.
Then, be truly selfish and choose altruism.