According to our scriptures (the Torah), God promised Abraham that his descendants would be "like the stars." While there have been brief periods in our history where we sought converts, by and large, the Jewish people are simply the descendants of Abraham and Sarah.
Abraham begat Isaac, Isaac begat Jacob, and Jacob (later called "Israel") begat 12 sons. The rest, as they say, is history, and a LOT of time.
We do still ACCEPT converts today, but we do not seek them; nobody will ever ask you to be Jewish, but you can be if you choose.