Fundamentally, a currency is backed by the resources of a country. The US Dollar is backed by the US GDP, the CNY by Chinese GDP, and so on. If US GDP increases, your USDs go up in value. Bitcoin however is backed by nothing. It has no intrinsic value per se, unlike currencies or stocks. It is valuable only because others think it is valuable. If tomorrow everybody thinks 1 Bitcoin is worth 100 USD, they are worth 100 USD. If everybody thinks they are worthless, they are worthless. It is the ultimate derivative, whose worth is derived from people’s perception of its value. Bitcoin is entirely unlike any currency or stock or bond known so far. The closest I can think of is precious metals.
However it is also unlike precious metals. Precious metals have a strong consumption demand. Gold for example has several industrial uses in addition to its demand in jewellery. Bitcoins have no intrinsic demand. Just limiting the supply of Bitcoins does nothing, as they have no intrinsic value. It is meaningless to talk about deflation or inflation of Bitcoins because nothing is produced in terms of Bitcoins.
Basically, Bitcoin is the ultimate fiat currency, whose value exists only because other people find it valuable. That makes Bitcoin an asset bubble. Forget stock market bubble, forget real estate bubble, at least those assets produced something of value. Bitcoin has no utility. It is the ultimate bubble since the Dutch tulip bubble. And one thing common to all bubbles is that they burst.