The whole Avengers initiative got started because Steve Rogers is in love with Bucky Barnes
Note: I am speaking as a stevebucky/stucky shipper, but my statement still stands that the whole Avengers initiative was created because Steve Rogers is in love with Bucky. Whether that is romantic, platonic, or otherwise is up to you.
In the events of the excellent (and underrated, in my opinion) movie, Captain America: The First Avenger, we see Steven “Steve” Grant Rogers make two decisions based on his love for James Buchanan “Bucky” Barnes that push other characters to make bigger moves which eventually create the MCU as we know it and the Avengers initiative.
The first of these decisions is that Steve single-handedly disarms a Hydra base because Bucky was captured. We would not have Captain America as we know it had Bucky not been captured. While some may argue that Bucky going to the war was the catalyst for the creation of the Captain America mantle, I believe that it was obvious that Steve was going to find a way into the war no matter what the cost. Now, whether this was because of the kind of person Steve is or whether it’s because he’s hopelessly in love with Bucky, the point still stands that Steve was going to find a way into Erskine’s study.
Now back to Steve rescuing Bucky from the Hydra base. Steve’s decision to fight the whole camp, rescue all the captives, and save Bucky is what made the military believe he could handle a corps of his own. This is when the real legend of Captain America was born. This version of Captain America was the one that inspired the heads of the military—namely Peggy Carter, Howard Stark, and Colonel Phillips. These influential people (and the Howling Commandos and the troops) now had someone they could believe in who was producing real results. This wasn’t a figure meant to inspire the country (although he did in a way). This was a figure meant to inspire the people on the front lines, who were fighting day in and day out. This is all fine and dandy... until Bucky falls out of a train.
The second decision that Steve made was to dive his plane into an ocean. Whether this was because he was heartbroken at the loss of the love of his life or because that is the kind of person Steve is, the world may never know. But I know that’s it because he was heartbroken AND he is that kind of person. With Steve’s death, the heroes left behind (Peggy + Howard + Phillips + the rest of the Commandos) had lost someone dear to them, but they also had a martyr. They had a figurehead whose ideals and principles inspired them. They wanted to uphold those and fight for what he believed in because he’d given everything. So what did they do? They created an organization with the goal to do just that, and they named it after what Steve used in battle. They named it SHIELD.
SHIELD, as we all know, went on to become the kind of organization that would hire a man named Nicholas J. Fury. A man who would have an idea. An idea to bring together a group of remarkable people, to see if they could become something more. To fight the battles that we never could.
tl;dr Bucky Barnes has always been (and will always be) the catalyst for Steve Rogers’ actions, and Steve's actions were the motivator for forming SHIELD, which eventually created the Avengers.
Thanks for coming to my TED talk.