Proverbs 28:21 (ESV)

21 To show partiality is not good, but for a piece of bread a man will do wrong.

It is sinful to show partiality instead of justice and fairness in our dealings with other people. Partiality is based on selfishness. It also lays a sinful snare for others, who learn from us to pursue their own selfish ends, and cause them to appeal to our partiality by sinful means. Partiality causes people to reject the just sovereignty of the Lord, because they look to each other, rather than to the Lord, for direction and reward.