Important verse of Srimad bhagvadgita -13

The steadily devoted soul(निष्काम योगी ) attains unadulterated peace (अगाध शान्ति ) because he offers the result ( सम्पूर्ण कर्म फल का त्याग करना ) of all activities to Me (lord); whereas a person who is not in union with the Divine, who is greedy for the fruits of his labor r(कर्म ), becomes entangle   (फँसजाता  है )...........5:12 

Attachment to the rewards to be gained from one's actions is the root cause for bondage to the perpetual cycle of birth and death in material existence. Lord Krishna presents this assertion with positive and negative examples. The aspirant for atma tattva or realisation of the soul who unmotivated by desires offers all actions unto the Supreme Lord attains moksa or liberation and everlasting peace. Whereas the person who performs actions never offering them to the supreme is contaminated with the blemish of selfish motives and the craving of rewards and is unable to relinquish(त्यागना) the conception of ego and proprietorship (स्वामीत्व ) thinking that they are the body and miserably failing by such mentality are bound tightly in material existence.

विशेष : 1) One's actions is the root cause for bondage

          2) Unable to leave the ego or I am the body concept

          3) Results in - unable to rewrite the perception of mind.. which is root cause  of attachment to the rewards to be gained viz; an archer eyes to shoot and he is focused on it , but as soon as he is attached to the reward for shooting bulls eye;  he is attached to the reward, and he see two goals now one  is to posses the reward other is to shoot right and he gets confused, such person never can become devotee, Thus no peace, no happiness inside.