Jacob has identified some very helpful criteria for assessing when you should "pull the trigger" on an index rebuild as well as a 'best practices' monitoring suggestion. To add to the reply to your initial question, Krishnrajsinh, the question of automating the rebuild is largely dependent on what you mean by frequently updated database. Is the table updated hourly, or just daily, or otherwise? Further, what is the need for index optimization (is this for OLTP?) In our shop we have some defragmentation jobs that are scheduled to run several times an hour, but that is not 'typical' within the broad scope of our SQL Server instances. So there are scenarios that can justify such an operation in order to keep online services performing optimally, if I am understanding your question. I also would echo Jacob's statement to review the nature of the index(es) themselves to see if they are both necessary and if they are efficient given the structure of the table(s).
Finally, you may have read through this, but the MSDN article on 'Reorganizing and rebuilding indexes' is not only thorough in its explanation but has some further helpful links in terms of index concepts and best practices:
commented on Dec 11 2012 8:19AM