Do you think that happened a lot, Mark saying something mean and then catching himself a moment later and acknowledging that he’d been mean (his version of ‘I’m sorry’, or at least Eduardo saw it that way)? I think it did.
Look at Eduardo’s face here in the 2nd and 3rd gif. His expression turns fond and expectant the moment Mark says his little dig, he’s not upset, not in the slightest, not for a second (and we’ve seen him dissolve into agitation to the point of hysterics over the chicken thing before), because he knows how this will play out. He knows that the part of Mark that says or does mean hurtful things might be a bit quicker than the rest of him, brain outpacing heart, always rushing ahead. He also know that Mark is his friend though, who’ll never actually mean to hurt him and will call himself out on his shit for Eduardo’s sake.
So when Eduardo freezes the account, because he just can’t, he can’t, he doesn’t know what else to do anymore, Mark says ‘maybe you were frustrated’, he says ‘maybe you were angry’, and Eduardo replies ‘I’m sorry’, and ‘I was angry’, and ‘I had to get your attention’.
And Mark says ‘I’m willing to let bygones be bygones’, because that’s it, that’s how they work that’s all there is. That’s the moment when Eduardo knows that everything will be okay and he can relax again. That is when Eduardo knows they are okay again, because Mark is his friend and Mark would never actually hurt him.
Look at that cute heart-warming gif set, at the love and adoration in Eduardo’s eyes at Mark’s words, at the way Mark pauses and turns to Eduardo, away from the party around them, to show how important it is that Mark knows that what he said was mean, that Eduardo’s feelings have and deserve his attention. This is why Eduardo never even saw it coming when Mark broke pattern and stabbed him in the back.