There is basically 2 reasons why this movie is the perfect sequel.
First: It is not about making money out of a very successful first film. Before Sunrise was not a success, it was an outstanding indie film that captured a rare moment between two persons. Which was an encounter that was very unlikely to happen between an american guy an a french girl while they are travelling. It was about the emergency of knowing the other person because you already know how it will end. The ending was a question that could be answered 3 different ways depending on your cynism or your romanticism. They would never meet again, they might but unlikely or they will definitly end up together. 9 years later the answer is given in Before Sunset.
Second reason: Every subject that was discussed in the first movie will be reopened by the two of them. The maturity and the desillusionment of both gives a depth to those discussions that were held 9 years before. The first movie add to the second, but the second also add to the first. Before Sunset is excellent. They talk about pertinent subjects. It isn't about USA versus France nor a man versus a woman, it is about two human beings living in a world of nonsense and trying to find their own way of living happily in it.