Derech Hashem – The Way of G-d 2:4:4
But G-d graciously saw to it that others could leave their root nation and join the family of Abraham if they’d care to 1. For G-d had made Abraham the father of converts 2, telling him that “all the families of the earth will be blessed through” him (Genesis 12:3).
If they don’t try to do that, though, then they’ll just naturally stay aligned with their root nation.
1 Thus, becoming a Jew comes down to leaving one’s own people and attaching onto the Jewish people (rather than only onto the Jewish religion). Like any other one, the Jewish people has its own ways and values, but rather than being molded by climate, circumstance, and the like, Jewish ways and values are rooted in Abraham’s dreams for his family of drawing close to G-d.
In fact, our sages pointed out that we went into – and continue to be in — exile in order to accept converts (Pesachim 87b).
See Ramchal’s insights into the place of converts in Otzrot Ramchal p. 149 and in Adir Bamarom pp. 353, 469.
Interestingly enough, a Jew can never leave the Jewish people himself (even if he “converts” or strays from Jewish practices or values), given that a Jew is always a Jew (see Sanhedrin 44a).
2 See Midrash Tanchuma, Lech Lecha 6.