Tobirama was born with a frown on his face.
He knows he's not the funnest person to be around. It's no matter — he doesn't want to be around people who value fun over practicalities, anyway.
Some people think his stoic attitude comes from a lifelong resentment of his brother's vastly superior skills. That is, of course, not true.
He feels every offense deeply. He can't seem to brush them off like lint, as he often sees his brother doing with ease. It's the only thing Tobirama's ever envied about him.
Everything about Hashirama is big. His brother has a big heart, a big laugh, big ideals, and is concerned solely with the big picture. By extension, only "big" things bothered him, if at all.
In contrast, Tobirama sometimes feel petty and small. A glance or a word is enough to bother him for days. This sensitivity is alternately amusing and worrying for Hashirama, who reads him in the way that only brothers can.
You feel too much, he booms, vibrating air molecules beyond the borders of Fire Country.
Look who's talking, he wants to snap back, but he doesn't because while Tobirama is ruthlessly pragmatic when it comes to doing the right thing, he is not cruel. Instead, he settles for a narrowing of the eyes and remembers his brother's face, fraught with shadows, after the creation of the Valley of the End.
He frowns.