Following virtues and merits of justice makes people eager to uphold it: To be loved by Allah, since Allah loves those who are just, Allah says: And if you judge between them, then judge between them with justice. Verily, Allah loves those who are just."
To be included in Allah's mercy and forgiveness on the Day of Judgement; Allah will shade the just ruler on a day when there will be no shade except His shade. The Prophet (PBUH) said: "Seven people will be shaded by Allah on a day when there will be no shade except His shade. They are: a just ruler…" al-hadeeth
To join Paradise in the hereafter; the people who act justly will be in Paradise in the company of Allah seated on platforms of light. The Prophet (PBUH) said: "Indeed the just and fair will be seated in the company of Allah on platforms of light. They are those who deal justly with their family members and in all affairs entrusted to them."
Preventing oppression; by prohibiting oppression; the opposite of justice. The Prophet (PBUH) that he related from his Lord that He said: "O my slaves, I have forbidden oppression for Myself, and I have made oppression prohibited amongst you, so do not oppress one another."
By informing Muslims of the evil effects and perils of oppression so that they would abandon it and thereby behave justly.