http://www.theatlantic.com/entertainment/archive/2014/07/why-most-tv-shows-peak-by-their-third-season/374750/

Mike Schur of “Brooklyn Nine-Nine” and “Parks and Rec” has an explanation: "Everyone’s favorite seasons of shows are seasons two and three, because you’ve had a year to get to know them, and then you’re still in the honeymoon period where you go, ‘This is great!’ And then after season three, everyone starts to go, ‘Eh, that show’s not as interesting as it was anymore.’ And it’s like, ‘Well you’ve been watching it for three years.’”
Read more at http://www.hitfix.com/tv-tattle#seYyTov7QCkXkSho.99

Some shows peak even earlier like Roseanne, ER, Friends, Frasier, The Golden Girls, a some later like Seinfeld, TBBT.

I do wonder if this has to do with that huge drop in ratings for The Cosby Show in season 4.

Personally, I think Friends really hit its stride once Monica and Chandler hooked up.

I liken long lived shows to the lives of people. The first couple of years the show and the person are in infancy, learning to crawl and getting to know what does and doesn't work. It's sloppy and goofy and a little disjointed.

The last few years the show (and the person) are getting on in years, and don't have the same energy or bite they once had. They've done it all, and a lot of what does happen seems like stuff you've seen before.

It's in the mid life of shows (and people) that the real energy and strength comes. They have learned from the past, and are still strong enough to hit their stride and do some real and compelling work.

ABC's Black Box is the only new network show that I've actually loved and wanted to watch in years. And the consensus is that it is going be cancelled. Wish there could be a 3rd season.

What happened to cable networks picking up shows?