Interest rates have fallen for 36 year.
Let's put that into perspective.

We’re currently living through the second longest bond bull market in recorded history, and the longest since the 16th century, according to a new research paper from the Bank of England.
...“The average length of bond bull markets stands at 25.8 years, and the range falls between 61 years (1451-1511) and 12 years (1718-1729). Our present real rate bond bull market, at 34 years, is already the second longest ever recorded,” Schmelzing writes.

Interest rates have fallen for so long that last year they reached their lowest levels in 5,000 years of recorded civilization.
When you have to go back to the ice age to find lower interest rates, you know that they can't fall much further. At some point the charts will show that an absolute bottom has been reached.

They have.

The yearly MACD chart -- moving average convergence and divergence -- for 10-year Treasury yields is getting very close to crossing the nine-year signal line. The two have met just once before, back in 1986 when the MACD’s slide below the signal indicated momentum was there for the sort of declines in yield that the current long-term bond rally has delivered. If the two cross in the other direction it would indicate the potential for sustained increases in yields.
Treasuries traders may need to get ready for the Halley’s Comet of chart points to appear. If it comes, it may be signal the end of the market as they and their forbears have known it.

And then there is this one.

“While everyone is watching the stock markets looking for a bubble to burst, or stressing about Europe, the real risk is probably complete mayhem in bond markets,” Blain said (see chart below).

The moment of truth has arrived for secular bond bull market! Need to start rallying effective immediately or obituaries need to be written.

— Jeffrey Gundlach (@TruthGundlach) October 24, 2017

Why does this matter?
For starters, interest rates have fallen for longer than a career on Wall Street.
There is no one left working on Wall Street that even knows how to trade a bond market in which interest rates are rising.
I think this is a very underappreciated dynamic.

Secondly, once-in-the-life-of-a-civilization low interest rates tends to encourage people to borrow money.
A loan at 2% interest may not make sense at 5%.
You can see the problem here.

Finally, there is something I've ranted about before - interest rate swaps.
What most people don't realize is that the interest rate swap market is far larger than the credit default swap market that scared so many people in 2008.
the global notional market for interest rate swaps was about $560 Trillion in 2014.

While using notional values dramatically overstates the actual amount of money at risk (unless there are systemic defaults), even a small fraction of that mountain of money dwarfs of the GDP's of the world.
What this means is that if interest rates were to rise, and rise quickly, it would trigger these interest rate swap contracts (i.e. someone would have to pay).

When interest rates rise, they rise across the board for an entire nation, and they may indeed help trigger interest-rate increases in other nations as well. Issuers have entered into trillions of dollars of contracts with Wall Street and other financial firms to cover their risk if interest rates increase. And if interest rates do increase substantially – they increase for the nation as a whole, in every state, for every borrower, and for every category.
So there is close to 100% correlation – the simultaneous losses being incurred could be likened to one firestorm spreading over the entire country.
And the reserves that were established to cover statistical assumptions of uncorrelated losses quickly disappear when it is correlated losses on a national basis that happen instead.
Wall Street and the banking industry remain as highly over-leveraged as ever. Just where are these hidden reserves of many trillions of dollars worth of capital to honor all of the derivatives contracts?
...
Do Wall Street and the other providers of interest-rate derivatives actually have the capital tucked away somewhere to cover the many trillions of dollars in increased interest payments for the entire nation (and the entire world), year after year, if rates were to rise by 5% or 10% or even more?
In practice, we know that Wall Street wasn't good for their promises on a little more than $1 trillion in subprime mortgage derivatives. There are more than 400 times as many interest-rate derivatives currently outstanding as there were subprime mortgage derivatives in 2008. There's no possible way. It would be an annihilation scenario, short of even greater levels of government interventions.

But now interest rates are scraping rock bottom. They have to go up, short of dangerous monetary experimentation or disastrous global deflation.
That means that probably sooner rather than later, interest rates will go up, and if they go up quickly (like, for instance, because of widespread defaults) then that mountain of derivative contracts will be triggered. A mountain that simply can't be paid.