The recent shift from skepticism to optimism may be the first sign of market euphoria that is typical toward the end of bull markets but has been notably absent thus far in the cycle.

The sell side indicator – which measures Wall Street's bullishness on stocks – has rebounded since an all-time low of 43.9 in 2012, according to Bank of America Merrill Lynch strategist Savita Subramanian. The indicator rose by 2.4 percentage points to 56.4 in June, its highest level since 2011 and less than 2 percentage points from its 15-year average.

"The indicator's expected 12-month total return is +12%," wrote Subramanian in Monday's note. "With the S&P 500's indicated dividend yield currently near 2%, that implies a 12-month price return of 10% and a 12-month value of 2673." The S&P 500 is currently trading around 2,423.

Source: Bank of America Merrill Lynch

The sell side indicator is a contrarian metric, implying that as Wall Street grows more bullish toward stocks, the more bearish the measurement becomes. The indicator is based on the average recommended equity allocation of Wall Street strategists as of the last business day of each month.