Bruce
Bruce Banner knew things could change. They could change drastically in the blink of an eye and then nothing was the same again. But he found it hard to swallow that when he had left Tony and Pepper, he had also left their perfect triad for good. When he came back from his well-needed trip to India (he needed to get out the box made of steel and class) the power couple wasn't a couple anymore.
That kind of change just didn't happen. Of course, Tony and Pepper had had their issues. And their fights. Bruce knew all about them, he was the balancing factor of the two not-so-well matching personalities. But Pepper and Tony loved and lusted each other which had always made everything right at the end of the day.
Until Pepper had left. Tony was in shock. It takes one to know one, an introverted scientist who wanted only selected people close to him. Bruce saw right through Tony's pseudo-relaxed bullshit. Tony was devastated and afraid Pepper would leave Stark Industries as well as she had left him, and Bruce had to admit it served him right. So often he had made her life stressed and miserable with his carelessness.
Too bad she had broken Bruce's own heart, too. She had married Phil Coulson. Coulson. What a choice! He didn't know what to think about it. He didn't want to think about it too much. He silenced his inner voice. He's not good enough for you. You should have taken me instead. Bruce could never have done it to Tony.
And now Tony was with Darcy. He loved her. Everybody loved Darcy. But Bruce would have never thought how well Darcy and Tony fit together. When she was with him they both flourished. He was happy to see them so happy together.
They were so alike.
When Bruce returned to the US for the wedding, he moved right back to Tony's, but soon out of there. One Tony Stark was a lot, two of them were too much. Being in the same room with them was like following to ping pong match with three balls.
He was truly happy for him. But at the same time, he was sad about what he had lost. The perfect triad. He knew him better than anyone. And he knew her, Pepper, well enough, that he doubted if she was ever going to be really satisfied with a guy like Coulson. He just didn't have enough of... anything.
Tony and Darcy on the other hand... he had his doubts on them, too. He knew what kind of snob he was (Bruce had never had strenght to stand against his will) and Darcy´s market shampoo which was now in Tony´s bathroom was either a beginning of an inevitable breakdown or the beginning of something totally new.
He left early back to India. Away from sadness and unfulfilled wishes he hadn´t dared to seek out for.
Tony was sad about his leave. "Already? I´d love you to stay." But he was used to him leaving. He had disappointed him so many times before.
Pepper, on the other hand, got it all. She saw right through him. She didn´t say anything aloud, of course. She was polite as ever. But he read her sad smiles, her face, and her body language. I´m sorry we couldn´t be more, but I will always love you. And he knew she knew he loved her, too.
He wished for a good future for all of them.