The S&P 500 has rallied on optimism

The S&P 500 is 1.4%, or 43 points, away from the all-time high it hit in September thanks in large part to this year's strong first quarter, which was the best 3-month start since 1998.

The big picture: The index has history on its side. Per LPL Research, when the S&P has stayed above its low point in December during the first quarter, it has ended the year higher 34 out of the last 34 times.

Between the lines: That's a strong track record, but stocks so far have been relying on some optimistic assumptions.

• "The market has priced in" some semblance of a U.S.-China trade deal that would ease tariff pain as well as an orderly end to Brexit, Randy Frederick, vice president of trading and derivatives at the Schwab Center for Financial Research, tells Axios. Investors are also optimistic about lower-for-longer interest rates.

The problem: These are all far from sure bets.

What to watch: The market is not pricing in an escalation of U.S.-E.U. trade tensions, Frederick says.

Go deeper: The stock market loves nothing more than to climb a wall of worry