This is just a classic example of something becoming dull over time. Adam Brody has nothing but fond memories of playing Seth Cohen on 'The O.C.' - entrancing all of teenage America in the process - but when you're so well known for playing one part, it can become an issue when people keep stopping you in the street to talk about it. That's what Brody's found, and it's gradually led to a sense of resentment about both the part and the show.
Brody is much older now, and he's moved on with his life. He's done plenty of work since then, and he'd rather talk about that. He says he's simply run out of things to say about being Seth Cohen, and he's sick of being asked about him. Try to bear that in mind if you ever see him out anywhere!