He was concerned with how he looked to others. He used special brushes and pencils to color around his eyes. He curled his hair. He put high heels on his shoes so that he would appear taller and perhaps, older than his age. Jacob was at fault for manifesting greater love for Joseph than for his other sons and for spoiling his as he did.
Partiality is always a form of injustice, and injustice is always wrong and causes evil.
We have seen this already in Isaac's greater love for Esau and in Rebekah's greater love for Jacob. Jacob loved him best because he was unhappy. But they refused to understands and treated him as an intruder. He spoke to them, but they did not answer, says the Midrash. They turned their backs on him.