"So no one told you life was gonna be this way/Your job's a joke, you're broke/Your love life's D.O.A.!" It is perhaps the catchiest theme tune in TV history, but by the final season of Friends there was a huge thematic dissonance between its lyrics and the show's newfound status quo.
Friends was envisioned as a snapshot of that time between university and adult life, when expectations are disappointed and your future is in limbo, and that uncertainty is reflected in the theme song. By season ten, almost everyone (except perhaps Joey) was in their preferred field of work, Rachel and Ross were raising their little girl, Monica and Chandler were married and looking to have kids and even the bohemian Phoebe was settling down.
The characters weren't the immature 20-somethings of the early seasons any longer; yes, they were still the titular friends, but that friendship wasn't the most important thing in their lives anymore. It became a show about the grown-up life which once seemed impossibly distant.