With our generation and in part, our parents’, the situational comedy "Friends" has been very popular and has inspired many similar shows (like "How I Met Your Mother" and "New Girl" to name a couple) since. Why is that? It’s a seemingly normal show.
"Friends" was a part of a generation of TV that aimed to understand real, everyday life, without so much of the glamor and idealism that had been a part of the media beforehand. The characters shared many of the issues they faced with other people from that same time, and from that same age group. Many of these situations are still relevant now as well, which is why the show still has plenty of influence years after it ended.
Take, for example, the episode when Rachel, Joey, and Phoebe are frustrated because they don’t have as much money as Chandler, Monica, and Ross (Season 2 Episode 5: The One with Five Steaks and an Eggplant) and they end up having to have an awkward conversation about it. That isn’t an issue that necessarily goes away; there are still plenty of people going through the same thing, and even I’ve been through something similar. The show tackles difficult situations like breakups, infertility, unemployment, marriage, and so on. The characters lives reflect the lives of everyday people, and the relationships reflect ones between normal people, with ups and downs and imperfections. Another reason that it’s been so successful is that it does all these things with comedy, which is almost like telling people that even in difficult situations it’s okay to find the humor and laugh.
I know the show isn’t perfect; it certainly has its flaws, which are mostly things that aren’t believable (for example, Monica’s apartment being rent controlled) and at times characters can be problematic. And it’s possible that I’m reading too much into the influence of one 90s sit-com. There is no denying, though, that there are several shows that have used the same character types and general plot-line--simply following several characters throughout the events of their lives--that have been as successful and loved if not more. If you haven’t watched "Friends" or any of the shows like it, I suggest that you do! I sometimes watch the re-runs as a source of comfort, or even when I just want something to laugh at.