Well, the short version is that countries lie to their people about how the real world economy works.
People believe they can have their cake and eat it too, which ultimately means borrowing more than they can ever hope to pay back for both individuals and governments.
This leads to debts that become impossible to pay back and asset bubbles like the stock market or housing markets which inevitably crash.
Once the economy crashes, governments scramble to get it back on foot, NOT so much to grow the economy again, but to prevent people from paying the price of being so stupid as to borrow more than they could afford or for voting in politicians who spend more than their respective countries can afford. To do this governments print off more money to pay for both the private and public debts, which is done through their central bank. THis money does not pay back the debts directly, but rather buys those debts off of the people who foolishly made them:
Mortgage Backed Securities
Asset Backed Securities
and I predict in the new future student loans
This money does NOT end up in the economy (which would cause inflation), but ends up simply recapitlizating (or "shoring up") the poor finances of the banks and lending houses that made such stupid loans. This, coupled with a harsh and recent memory of the financial crisis or latest bubble bursting, makes people reluctant to borrow this new money and so the financial system is awash with money that nobody wants to borrow.
In the end banks do not make money if they don't lend out their money. But people want their money deposited and held safely in a bank account. This means that banks would be providing a service (depository services aka "safe keeping") but have nowhere to lend it out to. Thus, instead of paying interest on those deposits, they start charging.
This occurs especially acutely, however, to banks or countries where they are deemed "more reliable" than others. ie - Switzerland.
Everybody wants to deposit their money in a Swiss bank because it is safe. Additionally, people want to own Swiss Francs as opposed to Euros (which would be destroyed by Greece), US Dollars (which would be tanked by a housing bubble or student loan bubble, or just massive government deficit spending) or Chinese Yuan (which would be ruined by corruption and their own debt problems).
Alas, Swiss banks and Switzerland cannot simply be everybody's banker, and they must charge (ie - negative interest rates) on their depository accounts.
And that IS about as simple as I can make it and that leaves a lot on the ground.