[Einherjar]

Why does every country, regardless of its economical reserves has some debts or deficit??

[Paradigm]

Because unless I'm mistaken, pretty much all of them use fiat currency.

[Einherjar]

what does fiat currency mean?

[Attila The Nun]

Irresponsible spending/tax cuts usually.

[Captain Charisma]

what does fiat currency mean?

Fiat: Arbitrary, so fiat currency is currency created out of air and not backed by anything solid.

[Adam_Smith]

Einherjar:
Why does every country, regardless of its economical reserves has some debts or deficit??

Paradigm:
Because unless I'm mistaken, pretty much all of them use fiat currency.

No, that's not it.

For example, even a very fiscally conservative government might borrow against confidently anticipated tax receipts in order to not needlessly delay the start of some useful project; e.g., disaster relief.

Indeed, when new funds are needed but tax revenue cannot be quickly raised, borrowing is the main alternative to "printing" fiat money. "Hard" currency would usually require in the short term that the government either borrows or does without.

What happens over the longer term, especially in democracies, is that politicans can use borrowed funds rather than raising taxes to finance government and leave paying to debts to their successors.

[Antiquarian]

This might sound childish, but I think that the reason is simple: governments want increasingly more power. If they are allocated only a certain about of money, they can spend more, as they please, and call it a deficit.