Back when Jews lived in the same communities for generations, they were obligated to follow the minhagim (customs) of their community. The classic example is that East European Jews held that one cannot eat dairy until after 6 hours following a meat meal, but that German Jews held that it was sufficient to wait 3 hours, and Dutch Jews wait only 1 hour. Now, Jewish communities in many countries include Jews from the four corners of the Earth and many people observe different minhagim.
Given that, how does a convert to Judaism know which minhagim are required of him? Would he look to the customs of his rav? Or would he look back at his geneology and adopt the customs of Jews from the area in which his non-Jewish ancestors lived? Could he choose the easiest or the strictest customs on his own?