All spoilers on this page are left unmarked. You Have Been Warned!
As already mentioned on the main page, second chances is the main theme of Avengers: Endgame, that exists in every choice or decision, moment or character arc conclusion present since the very beginning of the film. And the main message of the film is that, whether it means taking back what you've lost, or building something new out of what you have, you should always take a second chance when it comes around. For better or for worse, Avengers: Endgame wields its theme how Captain America wields his shield: as something to make him stronger, that he'll never give up.
Second chances are present since the very beginning with Tony and Nebula, where Tony tells her re: paper football, "we are now one apiece" and Nebula replies "I would like to try again". It's such a throwaway line, but something that sets the tone for the entire movie to come. We get back on earth, and we see that the Avengers are forming a plan to take back what they lost - a second chance to defeat Thanos. After Tony collapses to the floor, they leave for Titan-II. But Thanos isn't even worried when they arrive, and they soon see why: he destroyed the stones, the only possibility for a second chance to restore everything. Closing off the first act in a poetic way, Thanos remarks that "perhaps [he] treated [her] too harshly"...
Thanos and his complete failure to understand the meaning of a second chance is something that comes up a lot in the film. But, for now, there's the second act.
FIVE YEARS LATER
The Avengers have all moved on from the snap, to varying degrees, and all of them have changed. Tony has taken it as a second chance to start a family with Pepper Potts, and Bruce has taken it as a second chance to merge with the Hulk to become Professor Hulk. Steve and Natasha are desperately trying to stay as they were - Captain America, the inspiring symbol of the nation; and Black Widow, the mysterious agent who works in the shadows. Meanwhile, Thor and Clint have actually regressed into themselves, reverting back to jovial yet fierce warrior and hardened assassin respectively. Nat and Steve even have a talk about how she's "still trying to be better" while Steve says that they "both need to get a life". And then... enter Scott Lang.
It makes sense that Scott is the one to come up with the grand Time Heist - after all, his movies are about getting a second chance to become the hero in his daughter's eyes. And, together, they generate the Time Heist, during which they get to live through their lives a second time, but this time at different points in their stories, and with different motivations:
To bring them all home. Whatever it takes.