Sherlock is as cold as ever during a case. Facts, deductions, no time for niceties, for social interaction. He does not busy himself with things that are surplus to his work.
"The robberies were committed by someone inside the organisation, surely any fool with half a brain can see that?"
"The murderer was her sister. A case of jealousy. Murdered her to steal her husband. Nice and simple."
Sweeping in, observing, deducting, brain working at the speed of light. Facts, cold hard facts were Sherlock's preoccupation, and he had no time for sentiment.
At night, however, when the rain beats down on the windows of Baker Street, the doors are locked and curtains pulled, the words whispered make John's very soul swell to bursting. Entwined together huddled against the chilly bedroom air under the blankets, or hot, sweaty and passionate where all John is aware of is the feeling of bare skin on bare skin.
"John you are all I see."
"John you are the most beautiful soul..."
"I was not alive until I knew you..."
These private words, whispered into ears in secret in the night, are words meant for John only. These words do not come from thinking, from deduction, but from passion, from Sherlock's very heart, from moments of pure pleasure and lust and need.
These are Sherlock's private words, only meant for one other to hear.