How and why are people motivated by selfishness?

Selfishness motivates people because we want to better and benefit ourselves. When put under the pressure or the circumstance, we might become selfish at the idea of others beating us or being better or having more than us. Humans are typically greedy, which leads to this self-centered personality often times where we don't typically think at all about others. We might want to win something at all costs, be ahead or better than the rest, be the top dog - therefore we start to only be concerned with our own standing and benefits.