Arrogance can lead a person to over-assess himself in relation to others and be deluded about the reality of his abilities (the disease of ghuroor). This inflated self-image leads a person to believe he can do that which he is not equipped to do, much to the detriment of himself and those around him.
Another consequence of arrogance is that a person might be inclined to boast and brag when in the company of others (the disease of fakhr). Or it may lead him to be self-admiring, vain, and impressed with himself and his accomplishments (the disease of ‘ujb).
Arrogance can lead a person to be unwilling to forgive others and harbor anger and hatred towards them (the diseases of ghadab and bughd, respectively). It can also cause them to envy others because they feel that they more rightfully deserve the blessings that others have (the disease of hasad). And the list of diseases that takkabur is related to goes on and on – more than this post could ever encompass.
Thus, the disease of takkabur is indeed one of the worst diseases of the hearts and understanding how to purify our hearts from this disease is vital.