1. If one feels that he has earned it because of his own efforts, knowledge and capabilities and not because of the favour of Allah (swt) on him
2. It makes a person self indulgent.
3. It leads to show off and makes a person arrogant.
4. It is acquired by exploiting others.
5. It is not spent on doing good to the needy.
6. It is spent to make mischief in the land.
"O Allah, I seek refuge with Allah from laziness and senility, sins and debts and from the trial of the grave and the punishment of the grave, and from the trial of the Fire and the punishment of the Fire, and from the evil of the trial of wealth. I seek refuge with You from the trial of poverty and I seek refuge with You from the trial of the False Messiah. O Allah, wash my errors from me with snow and hail. Cleanse my heart of errors as You cleanse the white garment of filth. Put a great distance between me and my errors as You put a distance between the east and the west." (Bukhari:6007)
To know more, refer Surah Al-Qasas: 76-83