Just when you thought it might be time for stocks to pull back and for traders to perhaps rethink the low-volatility, steady march higher, the major indices blasted to fresh all-time highs yesterday. Just when you thought tax reform (or a lack thereof) and/or the political problems in Germany might become actual problems, traders decided to jump all over the buy buttons. And just when you thought that valuations in the high yield space might spill over into equities, even the beleaguered small- and mid-cap indices made a break for the border – and an impressive one at that.

So what gives? Why has the market mood flipped from worry to celebration? While we can never know for sure, it looks to me like the market narrative may be changing in a good way. And a little company named Goldman Sachs appears to have had a hand in the move.

You see, Goldman’s chief market strategist, David Kostin, has gone from being fairly cautious on his outlook for the U.S. stock market to something he’s calling “Rational Exuberance.”

Up until this week, Goldman’s year-end target for the S&P 500 stood at just 2400 – or about 7.7% lower than the venerable blue chip index closed Tuesday. And this is before the traditional year-end rally/window-dressing period to come. Oops.

But in a report to clients this week, Kostin suggested that there is plenty of room for stocks to run in the current environment. Assuming that tax reform passes and the economy doesn’t tank, that is.

Kostin writes, “Rational exuberance best describes our forecast for the trajectory of the S&P 500 during the next several years. Earnings drive stocks over time and should support the index rising to 2850 at year-end 2018, 3000 at the end of 2019, and 3100 by the close of 2020, representing a price gain during the next three years of 20%. Our price targets imply a modest expansion in forward P/E multiple to 18.2x at year-end 2018, a flat multiple in 2019, and a contraction to 18.1x in 2020.”