A/N: Canon not mine. Enjoy.
It was something he'd hum mindlessly while working, putting together a computer, writing a new program, fixing the coffee maker, he'd just default to it.
She didn't notice it before they moved in together, only after, he'd leave half finished projects lying around the house, and she'd inevitably come home and stumble across him elbows deep in a new project, humming away.
She'd tease him about it, hoping to embarrass it out of him, but he'd been doing it for so long that it didn't matter.
Soon after they began living together, irritation flooded her system when she heard him singing under his breath.
One time while watching the telly she heard him begin to hum and she turned to shush him but found him working on nothing, only looking at her. He smiled and she let the protest die in her throat.
Toward the end of their first year living together, she noticed something had changed in how or why he'd hum. He no longer hummed while working on something, but she'd hear him singing when he looked at her.
It took her a while to put it together, but when she did her affection for the song skyrocketed.
He'd hum when he was focused. He'd hum when he was around her.
She let it be, and instead of trying to embarrass him about such a simple song, she'd tease him that she'd make it the song they danced to at their wedding. He only grinned wider, grabbed her hands, and led her in a dance around their kitchen singing loudly,
"For he's a jolly good fellow, for he's a jolly good fellow, for he's a jolly good fellow, Which nobody can deny. Which nobody can deny!"