Private Investment Impact on Bond Yields From Japan to the US to Germany, we are currently seeing longer term bond yields falling at an alarming rate. 

I tune into a fair bit of economic commentary, and can often come to some sort of understanding of the factors that commentators say are at play in the current situation. But one I struggle with is the idea that weak private investment is a factor in declining yields. 

Can somebody please explain why this is the case? Is the idea that weak private investment leads to weaker GDP growth, which leads to weaker inflation, which leads to higher bond prices? Or is there something else at play here?
I think you've answered your own question. Not only would lower inflation be a factor in lower bond yields but also consider that a lower level of demand for loanable funds for private real investment means lower corporate bond yields. That view is cited in the following Larry Summers quote taken from this economics.stackexchange.com question on secular stagnation...


  Think about what the world was like when General Motors or AT&amp;T or
  Exxon or IBM were iconic companies. They were issuing debt. They were
  investing on a massive scale to expand capacity and to build networks.
  Now think about iconic companies of today like Apple and Google. They
  have more cash flow, fundamentally, than they know what to do with.
  And the result of that, of course, is an excess supply of savings.
  Another way to approach the question is to say, what's happened to the
  relative price of durable equipment, either producer equipment or
  consumer equipment, appliances? And the answer is those prices have
  gone way down. Well, when those prices go way down it means a given
  unit of savings goes much further.
  
  All of that, it seems to me, operates to reduce real interest rates