As with any long running show, it is expected that the creative juices will eventually start to run dry over time. Ok, so the show wasn't as good anymore. So maybe they should've quit when they were still ahead, like Seinfeld. But a lot of people honestly liked that show. At the time Friends came out, the concept was totally new. Now here was a bunch of cute people that weren't perfect! Unlike other shows of that period, their lives don't revolve around teenage angst. In the show, they're not rich. They struggle to survive. Despite their looks, the guys aren't babe magnets and the girls aren't fending off suitors left and right. In fact, they're sometimes losers. They often make mistakes. They're actually normal people that anyone can identify with. The comedy is OK, with a balance of verbal gags and slapstick. And there's always a story running subtly behind the jokes, making the individual episodes watchable independently but you still don't want to miss too many episodes else you'll get lost.
And yeah I'll admit that last episode was quite touching.