Bitcoin is a placeholder of a value in a blockchain ledger. The more you pump in the digital bullion the more it worth. Plus several factors that are not so vivid as capitalization. Taking into consideration a limited emission of the bitcoins.
Mined bitcoins: 16,690,400. All bitcoins: 21,000,000.
Current market cap devided by emitted ammount equals bitcoin value 135,914,600,512 / 16,690,400 = 8143$