Sawamura Eijun cares about his team. They're his baseball family, and he feels their frustration, their sadness, their regret and their joy so keenly inside himself that he just can't help but try to boost them up.
So he nags them about their warm ups and cool downs, about drinking and eating right, sleeping enough hours and putting on sunscreen. He screams praise, reminders and jokes on the field until his voice goes hoarse. He wants to win with them, but more than that, he wants to feel them around him, flushed with satisfaction and happiness.
He forgets in Seido that this team lives and breathes baseball, but when he thinks about it. Well, they need him too. Eijun sees the senpai relax just a little when they're yelling at him. He hopes that he'll be able to nag his teammates to take care of themselves enough that an injury like Chris-senpai's never happens again. He wants Haruichi to gain some confidence, Furuya to learn some social skills, and Miyuki to be a little genuine a little more of the time. So he nips at their heels like a Jack Russell terrier, barrages them with encouragement, reminders, congratulations, and warnings.
Because Seido is also his baseball family, and Eijun's heart is definitely big enough to love his 'home' team and his 'away' team just as much.