Why Are Interest Rates So Low?

Quite frankly, I don't really know and I can't make heads or tails of it. The current environment is a classic case of my habit of missing regime/paradigm shifts. The scales usually drop way after the trend or the event has manifested itself.

There are numerous reasons posited:

• Savings glut (this one is a mystery to me when you look at savings rates relative to history and trends...the argument is the savings glut comes from China/Russia/Brazil).
• Supply deficit (can't quite work this argument out; I think it relates to QE and lack of collateral).
• QE (combination of reduced supply and scramble for collateral).
• Financial repression (combination of debt overhang and distorted price signals from QE).
• Low inflation (yes inflation is low and has been trending down...let's call it declining inflation, but it is still positive and real rates are low). 
• Fears of deflation (I don't see deflation...commodities have taken it on the chin in the last year, but asset prices have gone through the roof and commodity prices are cyclical).
• Fears of market collapse/capital protection (maybe...but those Wally's have all missed the boat).
• Currency wars (fight to the bottom). 

I just can't understand why any investor would settle for a negative interest rate. This makes no sense to me at all. US rates look as attractive as ever relative to European rates. European rates are crazy. What are they telling us? Imminent collapse in the Euro? Maybe!

It should be noted, that you can't talk about why interest rates are so low in the US without taking account as to why they are so low (or even much lower) elsewhere.