Some changes happen gradually over time. It's hard to imagine Seinfeld not being about four jerks, but originally George (Jason Alexander) and Elaine (Julie-Louise Dreyfuss) were painted in a much more flattering light. If you look at the early episodes of Seinfeld, it seems like Jerry is set up to be the most self-centered of the four friends.
Alexander began portraying George in the spirit of the archetypal Woody Allen character. As he got further into the character, Alexander says he found himself saying repeatedly, "This could never happen to anyone, and even if it did, no human being would react like this." Every time he said that, writer Larry David would respond by saying that was how he had reacted in a similar situation. Eventually, Alexander started basing his performance on David more than Allen.
When he did this, the writers started to ask themselves why someone as smart and attractive as Elaine would hang out with these guys, and they decided to make Elaine just as dickish as her companions. As a result, the characters from later seasons feel like significantly worse people than the ones we met in the first season, but nowhere in the story are we told why.