Hello Sri,
Besides what Kilian asked you, I would like you to think about what you are asking.
The role system exists to define who can access what, in the application.
So, if a user has a role that is required to access a page, he can access the page, otherwise not.
If the user has a role that is required to perform certain action, he can perform that action, otherwise not.
Allow a user to "choose" which roles he want for himself is really weird, and does not make any sense at all, unless you are using roles for something that they are not meant to.
Let the user access everything (not have any role control) would have the same effect.
So, what are you trying to achieve?
Cheers.