Humanity has a horrible track record of determining what a "good person" looks like. In Athens, pederasty was a common practice, and those who practiced it were thought of as good and even noble for having sexual relationships with adolescent males. What a "good" person looks like to us varies with time, culture, and even emotional attachment. How often have you seen a friend with a total loser but, being "in love," they declare his "goodness/potential/etc?"
The mistake we make seems annoying at worst, but it's more subtle and scary than that: when we compare ourselves to other people to see how good we're doing spiritually, it gives us a picture which is totally wrong and useless. As 2 Cor 10:2 says, "For we are not bold to class or compare ourselves with some of those who commend themselves; but when they measure themselves by themselves and compare themselves with themselves, they are without understanding."
Measure yourself against the Word, not against what humanity or your feelings label as good. smile emoticon