Responses
Isayama often uses laughing to convey complex emotion of despair at the verge of insanity for the most crucial moments, like when Armin cornered Annie and asked her if she was really the female titan, and when Hanes was eaten at the end of 2 season. (for Annie, her laugh also showed her relief that she doesn’t need to hide it anymore)
It’s very expressive really. We can feel how distressed he is.
I know that it's not my place to say this, but maybe Eren laughed because he felt guilty of Sasha's death? Like he said, he treasure his comrades and wanted them to live a long life, but they end up dead the very next year because he himself rushed the plan over without discussing about it firsthand.
Just take a look at the time when Hannes died, Eren totally lose it and laughed the shit out of himself. If Armin had kicked the bucket too back then, I'm pretty damn sure that he would've laughed as well.
let's put it this way and choose 'crying' for an example. People cry because they're sad, right? And people do cry too when they're happy.