A defense mechanism
ואת-העם צו לאמר אתם עברים בגבול אחיכם בני-עשו הישבים בשעיר וייראו מכם ונשמרתם מאד
Command the people, saying: “You are passing through the territory of your brother, the children of Eisav, who dwell in [the land of] Seir. They fear you tremendously, and you shall be very cautious”
The book of Deuteronomy begins with Moshe recounting to the Jews their forty-year journey throughout the wilderness. They were about to enter the land of Israel, and Moshe was about to pass on from this Earth. Moshe wanted them to glean lessons from their failures and experiences throughout their travels, so that they’ll be better equipped for what’s to come. Towards the end of their journey, they began approaching the land of Seir, where Eisav dwelled. Moshe informed the Jewish people that the nation of Eisav feared the Jews tremendously. They should be very cautious as they pass through their land. In the end they weren’t able to pass through, so they had to circle around their borders. What lesson is Moshe giving the people by recounting to them this episode?
Moshe was teaching them an aspect of human psychology. When Reuven is afraid of Shimon, Shimon aught to be very careful around Reuven. Due to Reuven’s fear, whether it is justified or not, he will try every tactic in order to defend himself from Shimon. When fear is what is driving someone, there is nothing that will get in their way of protecting themself from harm. As such, Shimon aught to be very careful around Reuven, as now Shimon is in danger from the person who fears him.
We see this from Shaul HaMelech. The verse tells us that Shaul feared Dovid. As the rest of the story goes, Shaul relentlessly tried to kill Dovid, due to his fear and paranoia. Further, there’s a verse in Psalms which says מפחד אויב תצור חיי, protect my life from the fear of my enemy. Meaning, because my enemy fears me, he’s going to try everything in his power to hurt me. As such, the verse is describing a prayer for Hashem to protect from such an enemy. This is what Moshe was teaching the people. The nation of Eisav fears the Jews. That gives ample reason to be very cautious of them, as who knows what they might do to “defend” themselves.
This lesson that Moshe taught the Jews really applies to every Jew in every generation. For thousands of years the Jews have been in exile, scattered among the nations. The Torah is teaching us that the best advice for survival is not to arouse any nations’ fear. The second they fear the Jews, they’ll do everything in their power to oppress them. They’ll feel they’re defending themselves from this “foreign threat”. The Jews were the most secure in exile when they kept a low profile, not arousing the suspicion or fear of their neighbors. Something to keep in mind.
 Based on Be’er Yosef to Deuteronomy 2:4
 Devarim loc. cit.
 The Be’er Yosef feels he heard this from “HaRav HaGaon Bentzion Aryeh Leib Sizling zt”l “, a Maggid and Mechanech from Vilna (c. 1845-1915. Buried on Har HaZeisim)
 I Samuel 18:9: ויהי שאול עון [עוין] את-דוד. Cf. Rashi, Radak, and Metzudas Tzion ad. loc., who explain it as עין הרע, Malbim who says similarly that Shaul put his attention on Dovid, as he suspected him of being his successor, and Targum Yonasan, who translates it as כמין, which according to Jastrow seems to mean to lie in wait (although Radak sounds like it also means עין הרע). I couldn’t find any commentary that understands עוין like the Be’er Yosef
 Psalms 64:2
 See Devarim Rabbah 1:19