A verse and its Tafsir.
Allah the Exalted says:
“Lo! Allah enjoineth justice and kindness, and giving to kinsfolk, and forbiddeth lewdness and abomination and wickedness. He exhorteth you in order that ye may take heed.” (16:90)
Ihsan (spiritual excellence) transcends justice.
This is because justice is for a person to give what is due from him,
and to take what is rightfully his.
Ihsan, however, is to give more than what is due from one,
and to take less than what is rightfully one's own.
Thus, Ihsan is an addition to justice.
Striving for justice is an obligation,
while striving for Ihsan is a recommendation and a voluntary act of devotion.
It is for this reason that Allah the Exalted magnified the reward of the doers of good, saying:
“As for those who strive in Us, We surely guide them to Our paths, and lo! Allah is with the good.” (29:69)
And He said:
Indeed, Allah loves the doers of good.
Mufradat al-Qur'an by al-Raghib al-Isfahani.