Even if Monica and Chandler's romance wasn't planned from the start, some moments in the first four seasons of Friends hinted that they were meant to be together before they started dating. Monica and Chandler's romance was among the best storylines the show developed in its 10-year run. In fact, despite their hookup in London originally not supposed to go anywhere past a one-night stand, pairing them up gave Friends a surprising amount of fresh material from season 5 onward, from them initially hiding their relationship to starting a family together.

https://screenrant.com/friends-moments-monica-chandler-soulmates-prove/