Exst many reasons to say that Phineas and Ferb is a great TV show, but only need two:
1- It isn't repetitive because there's no build up:
This show is not story driven, it's comedy driven. Because of that, it doesn't get repetitive because the focus is always the characters and the story takes a backseat. We don't go into an episode to see if Candace will succeed, we go to see how she will fail. We know that Perry's going to defeat Doofenshmirtz, so we want to see what's his new scheme is and how Perry's going to defeat him. It doesn't create expectations, so it doesn't bore us. A good example is the Roadrunner. We know that the Coyote will fail, but we still enjoy it immensely. We never expect the Coyote to win because we don't expect him to. Same with Phineas and Ferb, we know that Candace's going to fail, so we don't expect her to win. This, in turn, makes the show anything but repetitive, since the focus is always the intricate comedy.
2- Good until the end:
Different from what a lot of people think, the quality the show only decreased slightly. When you think about it, no other cartoon has managed to stay so tight and consistent after seven years in the air. The comedy was still great, and even when they were going trough a lack of ideas, they refused to give up, which gave us great episodes even towards the very end. The decrease in quality was natural and inevitable, and it passed trough the harder phases a lot better than others shows in their seasonal rots. Heck, the two last episodes are considered one of the best of the entire show. This show avoided seasonal rot entirely.
So there it is. That's why we love Phineas and Ferb so much. It's a very consistent show that definitively left it's mark in the world of animation. I wouldn't be surprised if this show gets remembered as a classic in the near future. This a AWESOME SHOW!!! LONG LIVE TO PHINEAS AND FERB!!