There's partly a real trend here - when a show runs for many seasons, it gets harder for the creators to keep the tension up, and they start repeating formulas that worked before. But at the same time, you change too: you watch the first seasons with your jaw on the floor because everything's new, and then you get used to the world, the characters, the style - and nothing feels as fresh anymore. Usually it's a mix of both: there are shows that genuinely lost quality, and then there are ones that just stopped being a revelation for you personally.