Sometimes a Jewish person will ask you if you are Jewish because
1) They are trying to encourage other Jews to follow the traditions of Judaism. In Judaism you are not allowed to try to encourage people who aren't Jewish to follow Jewish practices. So, for example, they will ask you if you are Jewish, and if you say no, they will say "thank you" and walk away. If you say yes, that you are Jewish, they will then talk to you about lighting candles on Sabbath or something.
2) They want to ask you to do something that a Jew isn't allowed to do, but a non-Jew is. For example, on the Sabbath, a Jew isn't allowed to tun on anything electrical, so they might ask a non-Jew to come into the synagogue to turn on the lights and air conditioning.