We already know that Natasha Romanoff sacrificed herself to save the world in Avengers: Endgame. After all, it was either her life or the lives of billions who were snapped away by Thanos that would be on the line. However, it looks like there was another reason for Nat's decision and it may have been revealed in Hawkeye Episode 5. SPOILERS AHEAD, PROCEED WITH CAUTION!
In Hawkeye Episode 5, it was confirmed that Yelena Belova was still trying to find more Black Widows to be deprogrammed. As she catches up with two Widows over drinks, Yelena goes into the restroom. While looking at herself in the mirror, she suddenly turns into dust and as the room around her changes, suddenly reappears in the same room.
It's an awesome revelation of what happens to people who were blipped away by Thanos' snap. For Yelena, she was only gone for a few seconds but is confused when she is brought back to the same location after five years.
That's not the only thing we learned in Hawkeye Episode 5. Yelena being blipped away confirms why Natasha was so depressed after the Blip. Nat may have been looking for Yelena for all those five years and that explains why she was desperate to bring her younger sister back. Ultimately, Nat chose to sacrifice herself knowing that it would bring Yelena back in the end.
It's also interesting to note that Yelena was thinking of Nat right before she was blipped away. Nat was also the first thing she thought about when she returned five years later. Needless to say, the bond between the sisters continues to be a strong one.
Hawkeye Episode 5 is titled Ronin and also revealed the identity of the person who hired Yelena to kill Clint Barton. The episode is currently streaming on Disney+.