It usually happened when they argued.
Chell would be in the middle of scolding Wheatley for something silly he had done (because he could have hurt himself doing *insert crazy activity that only Wheatley would think up here*), when she would notice that he was trying not to smile (and failing).
And that, every so slowly, he was sneaking closer to her.
Chell would back away as subtly as possible, something nervous and ticklish bubbling up in her chest as she too fought the urge to grin. This was the part where she always told him to stop it, she was being serious and this wasn't funny.
In turn he would ask her, why, then, was she smiling?
They would both stand there for a moment, losing their battles to keep a straight face as their eyes met, and both began to laugh with a giddy sort of tension. After a few seconds of standing stock still, Wheatley would dart at her and Chell would dash away, effectively leading to a chase through the house.
Chell was infinitely more graceful than he was, which meant that she usually managed to get away or hide somewhere else in the house while Wheatley banged his leg on a piece of furniture or tripped and fell on his face. After that, though, he could locate her easily enough by the sound of her laughter. It was uncontrollable (and who could blame her? He had sounded like a panicked giraffe plummeting to his death), an adorable stream of schoolgirl giggles that escalated to near hysteria as Wheatley scooped her out of her hiding place and toted her away.
Of course Chell put up a fight; she always squirmed, and kicked, and batted him away, but this strategy would have been much more effective if she could stop laughing long enough for her "attack" to actually hurt.
Minutes later Wheatley would deposit her on the couch, where he then proceeded to kneel overtop of her and tickle her senseless. His fingers danced along her ribs; he blew raspberries into her skin; he adorned her face with kisses. This affair would continue until Chell was in tears and both their faces were flushed with embarrassment and laughter.
When they finally managed to calm down Wheatley would give her that dorky, lopsided grin as he pressed his forehead to hers.
"Love you."
She nudged him gently.
"I know."
"But you're still in trouble."