Excess liquidity causes asset price inflation which causes low yields and low risk premiums. Low yields and low risk premiums mean low expected future returns. If the excess liquidity leaves the system (perhaps because central bankers around the world increase interest rates) asset values "deflate" turning low returns into very low returns.
Example: The Bank of Japan holds short-term rates at 0% to fight its long-running deflationary problems (i.e. providing liquidity). Investors borrow 0% yen and invest in 4% treasuries earning 4% on almost no equity investment (i.e. very high return on investment). As investors exploit this attractive trade they bid up the price of treasuries and yields decline. If Japan removes the excess liquidity (by raising borrowing rates) the trade becomes less attractive and investors will begin to unwind their positions (by selling treasuries). As treasury prices decline, investors who borrowed to exploit this "carry trade" get squeezed and have to sell too, which puts more pressure on treasuries, etc. etc.
In this simple example the price of treasuries is bid up because of very cheap capital (the Bank of Japan willing to lend at 0%) causing prices to rise and yields to decline. The low 4% yield is already a "slim annuity" but eventually when the cheap capital goes away, prices fall too resulting in even worse returns.
Sam is saying the entire market (real estate, bonds, equities, etc) is akin to the treasury market in the above example.
