It wasn't until Friends season 4 that Chandler and Monica's relationship shifted from platonic — but the actors' chemistry meant that hints existed much earlier in the show's run. Played by Matthew Perry and Courteney Cox, respectively, Chandler and Monica were two members of the titular friend group. As such, they contributed to the show's stream of personal and professional hijinks in New York City. In the wake of the Ross and Rachel relationship having temporarily run its course, Friends season 4, episode 24, "The One With Ross's Wedding, Part Two" shocked fans by having Chandler and Monica sleep together.
During Friends: The Reunion, the show's creators (David Crane and Marta Kauffman) revealed that it was originally conceived as a brief fling. The initial plan was that they'd sleep together in London and, while the writers would have some fun with the aftermath, it would peter out soon after. Having recorded the scene in front of three different audiences, however, they took note of the visceral reaction the moment garnered each time. With extra time to mull things over due to the hiatus between seasons, the Friends writers realized that Chandler and Monica's relationship had a lot more potential. They would ultimately weave that potential into a love story that lasted the rest of the show — and lingered in fans' imaginations way beyond. While those behind the scenes insist that the evolution was planned only a short while in advance, Cox and Perry's natural chemistry quietly set up the romance all along.
Some theories suggest that Chandler's line in Friends season 1, about girls he likes "dating leather-wearing alcoholics", lined up with Monica dating Fun Bobby. While that was no doubt a stretch, rewatches made it clear that the pair's closeness and flirtacious bickering often blurred the line between platonic and romantic. Years before Joey enviously glimpsed the couple sat doing a crossword together in Friends season 5, episode 16, "The One With The Cop", the actors' comfort with each other meant the characters could often be seen cuddled up in each other's space. Equally, though it's often portrayed as stereotypical male labido and played for laughs, Chandler always made clear how attractive he viewed Monica. Similarly, Monica briefly watched Chandler sleep in Friends season 2, episode 19, "The One Where Eddie Won't Go" — which, in TV language, often denoted romantic feelings simmering under the surface.
Arguably, many of those - and the fact they knew each other's more intimate secrets - could still be construed as platonic. However, the affection that bled into the actor's expressions at such moments could equally be viewed as subconscious foreshadowing of a now-inevitable turn in their dynamic. Whichever way one leans, things came into starker focus in Friends season 3, episode 6, "The One With The Flashback". Towards the end of the installment, Chandler and Monica shared a moment that teased their friendship nearly bubbled over into something more at an earlier point. Though that was true for most of the titular friends in that episode, their moment felt less frivilous and instead rooted in something much more meaningful. As such, it resonated and stayed with fans more than the other such incidents.
By the end of Friends season 3, Chandler even broached with Monica the idea of being her boyfriend. This was the moment that woke the larger fandom up to the idea of the two being a couple. The signs really were all there prior to that, however — albeit then overshadowed by Ross and Rachel's more overtly dramatic love story. While the writers (and broad sections of the fandom) seemingly overlooked it, Chandler and Monica offered a more organic will they/won't they that steadily bubbled in the background. Such was why it immediately resonated with fans the moment Monica was revealed under the covers alongside Chandler. As well as surprising fans with an unexpected twist, Friends offered a sense of carthasis to something that had actually been building for multiple seasons before it was even planned.