Some may argue that the present, wider high yield spread is a function of the low Treasury yields, but historically there has been little correlation between the spread-versus-Treasuries and nominal Treasury yields.
Guy told me that the demand for high-yield debt is a function of the improving economy, declining returns for the debt market, and the fact that the weakest issuers were effectively shaken out by the last recession.
Equity returns before valuation adjustments are a function of earnings growth (which have their root in GDP growth) and dividend yield including stock buybacks.