Moshe Rabbeinu, in his closing address to the nation, reminds them of the special role they are to play throughout history.
We have a Pasuk that tells us that we are to be an עם סגולה, a unique and special nation. And the following verse says ולתתך עליון על כל הגוים, that you are to be "above" all of the other nations. It is to be לשם ולתפארת ולהיותך לעם קדוש. You are to be a name and adornment and a holy nation.
Based on the Talmud in מסכת ברכות, the תורה תמימה says that these praises are meant to show the importance of עם ישראל by remaining separate from the other nations. We become an עם סגולה by staying together and not mingling with the other nations.
We welcome sincere converts from all nations who come to us for guidance, rather than our going out to them and risking being tainted by טומאת העמים, the impurities of the nations.