This week's Parsha contains a frequently quoted Pasuk (Vayikra 19:18): V'ahavta Lreiacha Hakamocha. This is commonly translated to mean: " You shall love your neighbor as yourself." Rabbi Akiva said of this Pasuk: "This is a klal gadol (great principle) of the Torah" (Sifra on Kedoshim 4:12, Midrash Bereshit 24:8 and Yerushalmi Nedarim 9:4).
Chazal connect this to the saying of Hillel Hazaken: "That which is hateful to you don't do your neighbor. This is the entire Torah and the rest is commentary. Go and learn." (Shabbat 32a) He told this to a proposed proselyte (Ger) who asked Hillel to teach him the entire Torah "on one foot." The intention is that just as someone who goes against your word is hateful to you, so you should not transgress the Mitzvot of Hashem. This is the entire Torah; the "rest is commentary" for you to learn what not to do because it is hateful to Hashem (and, of course, to also learn what is proper to do for it pleases Hashem).
The Gemara (Avoda Zara 25a) notes that Sefer Devarim is called Sefer Hayashar (The Book of What is Right) because it contains the command in Vaetchanan (6:18): V'asita hayashar v'hatov, and you shall do what is right and good. The Torah Temimah explains that this Pasuk in particular was singled out to represent the entire sefer because it is consistent with our verse of V'ahavta Lreiacha Hakamocha, which is the fundamental lesson of the Torah.
We can understand that Rebbi Akiva would translate the Torah into the language then spoken in Yerushalayim (Maharsha on Shabbat 32a). But why did Rebbi Akiva change from the positive language of the Torah ("love your neighbor") to a negative emphasis ("do not do what is hateful")?
Chazal explain that it is difficult to command someone to love his neighbor as he loves himself for this is not natural. Also, Rebbi Akiva was being consistent with his opinion (Sifra Behar 5:43 and Bava Metzia 62a) that "Your life takes precedence over your neighbor's life," in the case where you and your neighbor are in the desert and there is only enough of your water for one of you to survive.
In a similar way, the Chiddushei Harim explains that this concept of v'ahavta Lreiacha Hakamocha is totally foreign to those who are familiar with the Torah, and Rebbi Akiva therefore phrased this question in a way that could be grasped by his questioner.
I would like to suggest that Rebbi Akiva was also being consistent with Rebbi Levi (Midrash Kedoshim 24:5) who said V'ahavta Lreiacha Hakamocha corresponds to the last of the Aseret Hadibrot, Lo Tachmod, thou shalt not covet, which is phrased in the negative.
The positive or negative emphasis also carries over into halacha. The Sefer Hachinuch says that one who violates the Torah by not being careful with another's money or by causing any monetary loss or pain to another also transgresses V'ahavta Lreiacha Hakamocha (note the negative emphasis). HaRav Yosef Dov Soloveitchik zt"l (Shiurim L'Zecher Abba Mari Vol. 1 page 57), citing the Rambam (Hilchot Deiot 14:1), explains that it is possible by fulfilling a Rabbinic mitzva of visiting the sick, consoling the bereaved, or gladdening the bridegroom and bride (note the positive emphasis) to simultaneously fulfill the Torah commandment of V'ahavta Lreiacha Hakamocha.
The Chasidic Master Rebbi Naftali M'Rupshitz (my great-great-great-great grandfather) notes that the Pasuk ends with the words "Ani Hashem," and explains the lesson of V'ahavta Lreiacha Hakamocha symbolically: When we place the two letters Yud side by side they represent Hashem. When you love your neighbor and place him beside yourself (Hakamocha), then "Ani Hashem."