The block reward halves approximately every 4 years, this increases the scarcity of Bitcoin over time as newly minted bitcoins will not enter circulation as quickly as previous years, this also means that after the halving, it is likely that the value of bitcoin will rise because older bitcoins inherit additional rarity due to this change. When you combine this with the gradually increasing block difficulty, it's clear why BTC is deflationary, and self-sustains its own growth.