Our sages said
"Do not allow yourselves to be made unclean by them, because it will leave you contaminated" (Lev.11:43): If you become ritually impure through them, you will ultimately be spiritually defiled.
"For I am the L-rd your G-d, and since I am holy, you must [also] make yourselves holy and remain so" (Lev.11:44): Just as I am holy, so must you be holy. Just as I am set apart, so must you be set apart. (Torat Kohanim, Shemini, Chapter 12)
A person should always remember that he must not make a partnership with a non-Jew or forge a covenant with him, for we find that Avraham made a partnership with Avimelech, and ultimately they forged a convenant with Avraham... Accordingly our sages said, "If someone makes a partnership with a non-Jew, it is as though he worships idols and forges a covenant with them. If he is a Torah scholar, he will show scorn for his Torah and profane the name of his Father in Heaven. He will squander his money, give his sons to the gentiles, slay his sons with the sword, exile them from their land and sell them to idolatry. (Tanna Devei Eliyahu Rabba,7)
Likewise we learn (Ibid.,8)
A person should always remember that he must not eat with a non-Jew at one table, for we find that Hezekiah, king of Judah, ate with a non-Jew at one table and ultimately was punished severely.
Our sages said (Sanhedrin 104a)
Because non-Jews ate at his table, he caused exile to his sons. This supports the words of Hezekiah who said, "Whoever invites a non-Jew into his home and serves him, causes exile to his sons."
Tanna Devei Eliyahu, Ibid., likewise teaches:
Whoever eats with a non-Jew at one table, worships idols and consumes idolatrous sacrifices. If he is a Torah scholar, he will show scorn for his Torah and profane the name of his Father in Heaven. He will squander his money, give his sons to the gentiles, slay his sons with the sword, and exile them from their land.
The prohibition against social contact with the non-Jew was expanded by our sages, as Rambam explains (Hilchot Ma'achalot Asurot 17:9)
There are other acts which our sages forbade. Although these prohibitions, themselves, have no Torah basis, our sages still forbade them to distance us from the gentiles lest we mingle with them and ultimately marry them. They forbade us to drink with them even where there is no suspicion of the wine having been used for idolatry. They forbade us to eat their bread or their cooking, even where we have no other fear of it being non-kosher.