As the Marvel Cinematic Universe grew over the decade-long "Infinity Saga" to include epic battles and cosmic threats from powerful alien gems, the franchise managed to remain grounded due to the strong relationships the larger-than-life superheroes had with not only each other but the loved ones in their lives.
Without some of the strongest relationships in the MCU guiding and influencing their actions, things might have turned out very differently for the various heroes who managed to survive the Infinity Saga. Of course, not all of the relationships in the MCU had the same dramatic effect on the MCU's Endgame.
Hulk & Black Widow
2015's Avengers: Age of Ultron surprised fans with the development of a relationship between Mark Ruffalo's Dr. Bruce Banner/Hulk and Scarlett Johanssen's Natasha Romanoff/Black Widow, though it never made it much further than the first sequel.
Black Widow seemed to have a calming effect on Banner since their first meeting in 2012's The Avengers, which extended to her skill calming Hulk down on the battlefield. However, despite Bruce's desire to start a new life together, Black Widow's dedication to the mission led to the end of their burgeoning relationship.
Thor & Jane Foster
2011's Thor banished the titular Asgardian (played by Chris Hemsworth) to Earth where he first met Dr. Jane Foster (played by Natalie Portman) as she was studying the atmospheric effects caused by the Bifrost.
Their immediate attraction led to a love that tethered them across the galaxy and kept Thor returning to Midgard to see her and to work alongside the Avengers. Unfortunately, Jane's work and Thor's responsibilities eventually led to the couple's separation, though the couple may return in the upcoming Thor: Love & Thunder.
Star-Lord & Gamora
Peter Quill/Star-Lord (played by Chris Pratt) was a bit of an alien ladies' man before he first met Gamora (played by Zoe Saldana) in 2014's Guardians of the Galaxy when they were brought together by the Power Stone in order to save the universe.
Their relationship took some time to develop as they first became shipmates and friends, though they eventually revealed their feelings for each other. Unfortunately, this was shortly before she was killed by Thanos to get the Soul Stone, and while a version of Gamora from the past has returned, she doesn't remember Quill.
T'Challa & Nakia
Not much is known about the romantic past between King T'Challa (played by the late Chadwick Boseman) and Wakandan War Dog Nakia (played by Lupita Nyong'o), though it's often discussed between the new King of Wakanda and his friends and family in 2018's Black Panther.
It's also clear that there are strong feelings between them as Nakia drops her mission to accompany him back to Wakanda and backs him during Killmonger's takeover. Their renewed relationship was shaping up to be a strong foundation of Wakanda's new direction to help the outside world.
Carol Danvers & Maria Rambeau
2019's Captain Marvel featured Carol Danvers (played by Brie Larson) as she began to put the pieces of her life back together following the accident that gave her incredible abilities and sent her across the galaxy without her memory to be manipulated by the Kree Supreme Intelligence.
When she returned to Earth with questions about her past, the answers still didn't quite make sense to her until she reconnected with her best friend Maria Rambeau (played by Lashana Lynch). Their strong friendship helped steer Captain Marvel back to who she was and set her on the heroic path to the Avengers.
Hawkeye & Black Widow
Jeremy Renner's Clint Barton/Hawkeye and Black Widow spent years working together before they each appeared in the MCU. Their partnership as S.H.I.E.L.D.'s top spies carried through to their membership on the Avengers, and their teamwork helped turn the tide of quite a few of the Avengers battles.
It was ultimately their deep-rooted love for one another that first brought Hawkeye back after the loss of his family, and then enabled his sacrifice of Black Widow in order to earn the Soul Stone in Avengers: Endgame.
Vision and Scarlet Witch
After Wanda Maximoof/Scarlet Witch (played by Elizabeth Olsen) lost her brother and Jarvis was transformed into the Vision (played by Paul Bettany) in Avengers: Age of Ultron, the two bonded as they joined Captain America and his new team of Avengers.
Vision began to understand his new humanity and emotions towards Wanda as the team was divided in Captain America: Civil War, though they had the chance for their love to grow in private while still working for separate teams. Unfortunately, Vision's death in 2018's Avengers: Infinity War is sure to have further repercussions in the upcoming WandaVision series.
Scott & Cassie Lang
Scott Lang (played by Paul Rudd) debuted in 2015's Ant-Man, where he vowed to live his life so that his young daughter Cassie (initially played by Abby Ryder Fortson) would be proud of her father. This led him to become an essential member of the Avengers throughout the Infinity Saga.
He was luckily inside the Quantum Realm when Thanos snapped away half the population, and he survived there for five years to find that his daughter Cassie had survived, but countless others had lost their loved ones. He brought his technology to the Avengers and helped them bring back those lost in Infinity War so they could be reunited like he was with Cassie.
Iron Man & Pepper Potts
One of Captain America's heated insults to Iron Man during their confrontation in The Avengers was that he wasn't "the guy to make the sacrifice play," which may have been true of his life before he became Iron Man and turned his life around.
Because that moment also made him realize how much Pepper Potts (played by Gwyneth Paltrow) meant to him, and their relationship would help guide his decisions throughout the entire MCU franchise. The biggest surprise of the five-year-jump in Endgame was their daughter Morgan Stark, who proved to be his final reason for sacrificing himself to stop the threat of Thanos.
Steve Rogers & Peggy Carter
While fans initially hoped Steve Rogers/Captain America (played by Chris Evans) would find happiness with her niece Sharon, it was clear that Peggy Carter (played by Hayley Atwell) was Rogers' one true love after their short time together in 2011's Captain America: The First Avenger.
It seemed that the time-crossed lovers would be forever denied their date Captain America missed when he sacrificed himself to stop Red Skull, though Endgame revealed they found there happily ever after when Steve went back in time to marry the woman he loved after finishing his nearly century-long tour of duty.