Evidence
Harry and Hermione, as an Auror and an administrative member of the Department of Magical Law Enforcement, respectively, are ruled by evidence. It controls their every work at action, even permeating their relationship. They search for evidence endlessly; a glance there, a brush of the skin here, a tone of voice. Sometimes they search a little too hard, missing the obvious and overanalyzing the minute. Sometimes they don't search well enough; signals go over heads and below radars.
Sometimes, though, the evidence is unnecessary and they make do with simple words.
"I love you, Harry."
"I love you, too, Hermione."