Why does Coinjoin need multiple participants to increase privacy? Coinjoin is a method that make a transaction with other transaction of
  other parties to hide the link between txin and txout.


In case of this, why we need others transaction? We have many bitcoin address since we are using HD Wallet(BIP32) so I think we don't need to wait other bitcoiner to participate into shuffling for making a transaction. we can just give our address set.

Is there a special reason that enforce to combine mine and others stuff?

Sorry for my English.
The only advantages of doing a coinjoin is that the recipient of your funds cannot see the source of your funds (and the sender of funds to you cannot see your future transactions). If you do a coinjoin with only your funds, any branch they follow along this trail will lead to a source (or destination) of your funds.

Every source you include in the coinjoin will thus be exposed. Your recipient, of course, won't know that they're all of your sources, but if they're keeping a log of addresses that are likely tied to your identity, now all of those addresses, and only those addresses, will be in their log. If you instead do this with other people, it muddies such a log with false leads.

Similarly, the sources of your funds will all have logs of that destination of funds. Also, they can follow the chain back and deduce that your address is likely tied to another address if you have multiple sources, and they will all have an list of your other addresss. Again, they won't know that the other address are necessarily your address, but they actually will be, and so their log will contain accurate address linkage data.