You can say "Speak for yourself" when you have a different opinion
Sometimes, another person states an opinion, and you do not agree. When that occurs, you can use the phrase, "Speak for yourself." My guess is that the full sentence would be, "You are speaking for yourself only." It means that you are telling someone that the opinion that he stated is his, and not yours. It is, in a way, a relatively neutral way of saying that you think/feel differently.
Examples
A. Everyone knows that actors on Boys Over Flowers are all cute.
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