Back in 2012, just after The Avengers came out, I was talking to a friend about it and he told me he hated the movie. He didn't understand what Thor and Loki were fighting about, he didn't get what the deal with the Tesseract was, he didn't know what was going on with Black Widow, or what SHIELD was supposed to be. When I explained that all that info was in the previous movies, he replied, "What? I have to watch other movies to understand this one? That's stupid. This movie should explain everything I need to know."
A couple years later, I was marathoning the MCU films with my step-mom's family. Cumulatively, they had seen about half of them. When we finished Iron Man 2, my aunt said, "so now we watch Iron Man 3?" I said, "No, now we watch Thor, Captain America, The Avengers, and then Iron Man 3." She was puzzled. She wanted to watch Iron Man 3 and didn't understand why we had to wait.
Earlier this year, my friends and I were making plans to go see Captain America: Civil War. Everyone was excited, but one friend asked, "I haven't seen Captain America 2, is that bad?" I was honestly astounded that he was excited enough to see it opening weekend, but hadn't bothered to see the previous one. I explained that not only did he need to see The Winter Soldier, he also needed to see Avengers: Age of Ultron and, if he had time, Ant-Man.
All these cases are people coming from the same general assumption; that each Marvel film, even the team ups, is self-contained and the only thing that matters is watching the individual character lines in the right order. That's been the Hollywood model for superhero films up to this point, but as the loyal Marvelites know, it's no longer the case. The entire Marvel line is one giant series. Each one contributes to a larger story that feeds into the next one. You wouldn't expect The Lord of the Rings: The Return of the King to explain what the ring is and how it works, because that was already explained in the first film which you presumably saw. Likewise, the Marvel films will always assume you've seen the previous installments.
