Ignorance

There are three major problems with every human being - 

(1) One is of inadequacy - A person always feel that he is inadequate or incomplete - he wants more comfort, more wealth, more rights, more knowledge, more everything, but he never feels satisfied. In other words his desires are never-ending.
(2) The other is of plenty of choices - He has so many choices still he wants more choices and becomes choosy in enjoying them, and
(3) The third one is ignorance - His knowledge is always less or incomplete. However much information or knowledge he attains, he always feels that he needs more knowledge.

While other beings or living creatures do not have these problems, these are the causes of sorrow and discontentment and take him far from God. Since the origin of these problems is directly related to brain given by Bhagavaan to him, these problems cannot be solved generally. It takes really a Herculean effort to achieve that. That is what Geetaa is trying to achieve.

That is why it teaches to be a Sthit-pragya (a detached one) so that a man can restrict his desires (no inadequacy), and be happy what is available to him (no choice)