WARNING: The following article contains major spoilers for Avengers: Infinity War, in theaters now.
Avengers: Infinity War tore the Marvel Cinematic Universe apart, leaving the various heroes in a state of disarray. Thanos, as he promised, proved to be too much for them, acquiring all the Infinity Stones, and then harnessing their combined strength via the Infinity Gauntlet to kill off half the inhabitants of the galaxy.
This occurs in an epic finale in Wakanda where he literally commits this act with a mere snap of his fingers. As he does so, we see a bunch of superheroes crumbling to dust and fading away in one of the most shocking moments ever in pop culture cinema. As the movie winds down, one survivor is visibly traumatized by what has transpired, perhaps mores than the rest Steve Rogers (Chris Evans).
He sits on the ground next to Bucky Barnes' ashes, with Thor, Hulk and Black Widow all shellshocked an in stunned disbelief. As War Machine asks what's going on, Steve replies, "Oh, God!" -- an exclamation marking the first time he's ever truly felt defeated. However, with Avengers 4 hinting at being a search for answers, it's clear that the MCU needs him to wield the shield once more as a symbol of hope.
Steve renounced the mantle in Captain America: Civil War after he came to blows with Iron Man over the Sokovia Accords, becoming an enemy of the state and giving Tony Stark back the shield his father created. He no longer believed he was fighting on the right side because he saw the Accords as something counterintuitive to freedom, and more so, he was still loyal to a brainwashed Bucky.
But in the years that followed, he still protected innocents, working covert missions with the likes of Falcon and Widow as a team of "Secret" Avengers. Given that he also left a phone for Tony to call him whenever trouble brewed, he did show he was always there if needed, just not in an official capacity as Captain America. however, Infinity War's ending creates the perfect space for him to wield the shield again because, let's face it, the MCU needs inspirational leaders after Thanos' mass murder.