Are low interest rates dangerous?
If by dangerous it is meant they add possible downwards volatility of assets, that could easily be argued for as:

$ PV = D / (1+r)^t$

Where D denotes any future income. This is especially the case if the reason for the low rates is very easy monetary policy, as such policy cannot be maintained and thus the rate is not equilibrium rate and must rise, causing a crash in asset values when that happens.

Central banks determinate the interest rates in the short run, their policies are targeted to lower rates, thus a low rate. The other reason is that of supply and demand. There is not much demand for credit (no profitable ideas, no more room for consumer credit) and on the other hand the supply of credit is significant due to central bank action. High supply and low demand equals a low rate.