Looking for peers in a private run blockchain using geth

Question

I've installed geth on ubuntu 16.04 (xenial). After creating the following genesis.json file:

{
  "config": {

        "chainId": 4568,

        "homesteadBlock": 0,

        "eip150Block": 0,

        "eip155Block": 0,

        "eip158Block": 0

    },

  "alloc"      : {},

  "difficulty" : "0x400",

  "extraData"  : "",

  "gasLimit"   : "0x7A1200",

  "parentHash" : "0x0000000000000000000000000000000000000000000000000000000000000000",

  "timestamp"  : "0x00"

}

I created an account (same as eth.coinbase) and launched miner.start(). The hashrate is >0 and I receive the following message "Looking for peers" for more than 2 hours. I don't understand:

• why do I need to look for peers in a privately run blockchain?
• why is it taking so long ?

Best