I don't know, I never tried. I don't want some Rabbi slicing off the end of MY dick, thank you! But I think it may be because they don't want people to adopt their religion unless they are absolutely certain that's what they REALLY want. Being a Jew is more than adopting Judaism. A lot more!
answer: to ensure that a seeker is sincere and determined. Jews have ALWAYS accepted converts, even when it meant a death sentence for the convert, the rabbi converting them and sometimes the entire Jewish community from their Christian or Muslim neighbors. Ruth in the Tanakh/OT is a famous convert (without whom there never would have been King David, Solomon, etc).
There is more to converting than just announcing you believe what Jews believe and you're set. It takes a minimum of a year to convert because there is a lot to learn to be able to participate in the community fully and in the religion. Seeing if someone is sincere or easily dissuaded helps the rabbi determine if the potential convert is going to stick through the long process and if this is where they are called.
I don't know for certain, but I'm going to guess that they want to be sure that converts are absolutely serious, and have considered any negative consequences already.
Because of the history of anti-Semitism, the Jewish community is a tight-knit group connected on many levels above and beyond the technicalities of religious beliefs. So read on, and learn all there is to becoming an FOWA (Friend of Woody Allen).
Read more: How to Go About Converting to Judaism | eHow.com http://www.ehow.com/how-to_4845383_go-converting-judaism.html#ixzz16QdXSp9t
IF a Rabbi tries to dissuade you it is to make sure you are serious enough and have the determination to continue on. It is not really the common practice nowadays.