There is a moment, a tiny, barely visible moment in a life, when something inside a person shifts so irrevocably that it stops their heart, and they fall in love so hard that there's a tiny fear of hitting the ground and having no one to pick up their liquidised guts.
The Doctor can recount that moment exactly.
The moment he fell in love with Rose Tyler.
Back on Earth in the present day it was a Tuesday, so a pile of dirty clothes sat in the corner of the TARDIS, waiting to be taken back and washed in a high street laundrette with clinical smelling washing powder by a woman with hoop earrings that were a little too big and a ponytail that was a little too tight. But outside, the Doctor and Rose laid on a beach with their hands joined on the sand.
"The sea's pink."
She said it so quietly that he turned to look at her in concern, only to see her shoulders shaking with laughter and her eyes scrunched up into that smile he'd come to adore. The Doctor couldn't help but join in.
"Is there a problem with that?"
Rose turned to face him, no longer laughing, but the cheesy grin still etched on her face as she gazed at him. He quietened as well, and rolled onto his side so their faces were inches apart, as they'd been so many times before, always so natural, so right.
"No. I like it. Nah, I love it."
He knew she wasn't just talking about the sea.
He rubbed his thumb over hers a little, just enough to bring her eyes down to where their fingers were intertwined and make her giggle faintly. She looked back at his face and caught him staring down as well, his mouth slightly parted and his lips turned up into a smile, before his eyes once again flicked up to her face, boring into hers with the ever present adoration that tattooed itself onto his retinas every time he was in the close vicinity of Rose Tyler.
She reached out with her free hand and brushed back a strand of hair that the sea breeze had blown into his face, and there was such tenderness in the movement that the Doctor almost gasped.
That was when his heart stopped.
Every single cliché Earth romcom he'd ever been forced to watch shot through his mind in some light-speed montage, and he suddenly realised that the over dramatics he'd laughed about were all true.
Time slowed down, so he could see every fleck of colour in her irises, every little blush in her cheeks, every glint of her teeth from behind her wide smile. He couldn't draw breath, couldn't blink, couldn't look away. To hell with looking away, he thought, I'll stare forever if I can. All the noise from the rush of the tides and the blow of the wind drowned away, only to be filled by the sound of his own blood humming around his body and his hearts pounding in his chest.
And then, as quickly as it had come, the feeling left. The waves broke around their feet in normal time again, and his blood receded back to a normal state, but he still gazed at Rose as though he were a man in a desert who'd just seen a waterfall. She brought her hand down slowly and her eyebrows knotted together in a frown as she studied him, his lips wordlessly mouthing and his eyes popping slightly, darting across her face.
"Doctor? Are you okay?"
He mustered enough of his brain cells to nod dumbly, and while it clearly didn't convince her, Rose relaxed enough to snuggle just a little closer and he had to physically fight the urge to wrap his arms around her waist and pull her closer, shelter her from the harsh sea air. But instead, he started slightly and sat up, then twisted so he was looking down at her, only to see Rose grinning like a lunatic back at him.
"You look like you've seen a ghost!"
He shook his head, a little more conscious than before, smiled cheekily back at her and pulled her to her feet. She laughed quickly, loud and cheerful before he pushed her through the doors of the TARDIS, his hand a little lower on her back than before.
"Come on, your mum will go spare if you don't get the washing to her!"
As he closed the door behind him, Rose ambushed him from behind and jumped on his back, dragging him down to the floor with a yell.
"I liked the beach!"
"Well, if you hurry up and sort your manky clothes out then maybe we can come back."
With a look of mock hurt, she stood up and dusted herself off, before dramatically flouncing over to the pile and folded it quickly, a beam plastered across her face as she did so. The Doctor raised himself from the floor and leaned his head back against the door, gazing at Rose. Even the most mundane tasks were fun to watch when Rose was doing it whilst singing along to whatever song was in her head at the time.
God, he loved her.
There it was. The simple notion that he, the Doctor, was completely and indisputably in love with Rose Tyler, a human who would one day grow old, and would one day die. He released a shaky breath he didn't know he'd been holding as he processed that. One day, Rose Tyler wouldn't be there. She'd be a memory kept alive by his thoughts and the thrum of the TARDIS.
He didn't want to look up, but as he did, he saw her dancing around the console, belting out Elvis' 'I Can't Help Falling In Love With You' and was filled with such an inexplicable warmth that he decided, right there and then, that the happiness he felt every time he looked at her was worth the inevitable heartbreak.
It had to be.