Something that isn’t talked about nearly enough in the fandom is these two scenes.
"But wait" you say "People talk about them all the time"
Well, you’re right, they do. But not in the way I mean. You see these scenes and your first reaction is an emotional one, all you can focus on is the fact that Marco is dead and that his friends are so obviously mourning him. It’s sad, tragic, and that’s what people focus on.
When you look past that, and look at the context of the scenes you see the clear difference in them.
It’s no mystery to anyone that Jean and Eren are foil’s of each other. Meant to be similar and different at the same time. Many devices are utilized in both the anime and manga are utilized to illustrate this. One of the most important is Marco, or more specifically their relationship to him.
This scene, in particular, is really important. Because of the way they’re mourning Marco.
Put simply:Jean is mourning what was and Eren is mourning what could have been
Jean sees Marco as a trainee, and Eren sees him as a scout.
This speaks volumes about Jean and Eren and their different world views.
Jean lives in the present,what’s happening, how do I survive, how do I make it to tomorrow. He joins the army with the goal of being safe.
Eren lives in the future, where am I going, when do I get to see the world outside, how do I defeat the titans. He joins the army with the goal of making the world a better place.
Similarly, his death effects them in different ways.
Jean joins the Scouting Legion for the sake of a better future, while Eren makes more careful decisions in regards to immediate consequences.
While alive Marco, both on screen and off screen, insists Jean would make a great leadre. He is a pillar of support that he desperately needs. On the flipside, Eren insists that Marco would be a good leader when Marco has doubts about himself.
Again, this illustrates another pivotal personality difference between Eren and Jean. Jean needs support while Eren gives it. (This is also illustrated by their relationship with Mikasa. Eren demands Mikasa allow him to look after himself while Jean wants Mikasa’s help to survive during Trost)
Circling back to the scene dipicted above, Jean is remembering the words Marco said to support his decisions in Trost while Eren is picturing Marco in front of him and wearing the wings of the freedom (presumably in a role of leadership since he’s in front and looking back while all the cadets are facing forward)
tl;dr:these scenes are about more than just Marco being dead