Strong fundamentals haven't quieted investor worries. Corporate earnings are surging, economic data remain solid, and oil prices are tumbling after the interim U.S.-Iran agreement. Yet the S&P 500 has posted double-digit gains for three straight years and is up 9.6% this year, fueling skepticism about sustainability.

Many investors point to historical patterns: big rallies often precede market slides. While the index's forward price-to-earnings ratio has declined as profit expectations outpaced price gains, valuations still look stretched against Treasury yields. The disconnect suggests the market may be pricing in perpetual growth.

With Space X shares soaring and big tech companies plowing hundreds of billions into artificial intelligence, momentum feels vulnerable. When stocks trade at premiums to risk-free rates, there's little buffer for disappointment. The current setup leaves investors betting on continued economic magic.