Tantrum
Tony liked having Bruce in his lab. Loved it, even. Pepper had thought he'd gone completely insane when he said this, because Tony hated having people in his lab when he was trying to work. But Bruce was different. It was nice to have someone other than JARVIS to bounce ideas off of. Bruce was a genius. Not as smart as Tony, of course, but he (almost) never slowed him down.
It wasn't until Clint fraking Barton started hanging around the lab with his newfound BFF that Tony started to get frustrated.
"Something the matter, Tony?" Bruce asked when Barton left the lab.
Tony grumbled something incoherent.
"What was that?"
Tony looked up and pulled up a fake smile. "Barton's sure been hanging around a lot, hasn't he?"
"I guess so," Bruce shrugged. He paused. "It's not a problem, is it?"
"No, no, just seems a little weird. But it's fine." He returned to his work.
"This is totally bothering you, isn't it?"
"I told you, it's fine."
Bruce raised an eyebrow at him. "It's not like you to bottle things up like this. What is it?"
Tony looked back up at him. "You're very chummy with Hawkass," he said, as if it explained everything.
Bruce stared at him for a long moment, an amazed smile starting to appear. "You're jealous."
"I am not," Tony said, outraged at the though.
"You are. You're jealous because I made a friend without you."
"Am not!" he exclaimed, before storming out of the lab, right as Clint was walking it. Tony made an irritated sound and slammed the door shut behind him, shattering the glass, then stomped up the stairs.
"What's wrong with him?" Clint asked, passing Bruce a cup of coffee, and setting the other down at Tony's workstation.
"Just another temper tantrum," Bruce said, sipping his coffee. "He'll be fine."