Grief

courtesy: http://prramamurthy1931.blogspot.in/2010/11/quotable-quotes-from-valmiki-ramayana.html

शोको नाशयते  धैर्यं शोको नाशयते श्रुतं।

शोको नाशयते सर्वं नास्ति शोकोसमो रिपुः॥

Shoko naashayate dhairyam shoko naashayate shrutam

Shoko naashayete sarvam nasti shokasamo ripuh             

 2.62.15

Grief destroys one's courage.  It destroys one's learning.  It destroys one's everything.  There is no enemy greater than grief.

[Context: These are the words of Kausalya.  She had spoken harsh words to Dasratha for sending Rama to the forests.  She repents and says that she spoke thus because of  her intense grief on Rama being sent to the forests]