No I don't think you need to share every secret with them because one day they may not be your best friend anymore.

We sometimes need a person to confide in; tell a person that we trust about something which is secret or private, or confide our worries to that person... However, some people 'have a big mouth' (It is an idiom: people who are gossipers and they tell secrets). Others may reveal our secrets because they are simply unreliable or because we don't have a close and friendly relationship with them. That's why we should never rely on those who don't keep secrets, those who betray a confidence, or those who are not so close to us... But certainly there are some people with whom our secrets can be safe... 'They carry the secret to the grave' :) ... 'They don't let it go any further' because they are simply trusty and reliable... and because they are really close to us... However, we sometimes like to hide our feelings, ideas and thoughts from others...

Hello , A correction of your question: "Is it necessary to share every secret with your best friend? What is your opinion?" You could also say: " Is it recommended ...?"

i don't like to share secret, becouse secret between 2 is not secret

This is each one's stuff. Anyway, i think is better don't to tell it all about oneself. It's pretty boring! Besides, some things must be only for oneself. ........ "Don't ever tell anybody anything. If you do, you start missing everybody" ~J.D. Salinger, The Catcher in the Rye~