Rashi (Bava Kama 63b, DH Hekesha) writes that because verses are written near each other in the Torah, it is possible to compare the Dinim stated in these verses. Tosfos (Zevachim 47b, DH Eilu) writes that sometimes, because they are written near each other in the Torah, it is as if the Halachos of one Mitzvah are written also in the other Mitzvah.

The Teshuvos Chavos Yair (#203) writes that some Hekeshim are very strong and based on an extra word or letter in the Torah, while other Hekeshim were derived by Chazal who learned one thing from another through a Sevara (logic).

Why, though, is a Hekesh preferable?

In fact, Tosfos (Berachos 50a, DH Ela) writes that the Gemara here does not literally mean that a Hekesh is preferable to a Gezeirah Shavah, but in fact that they are equal. When the Gemara states that a Hekesh is Adif, it really means that it is just as important as a Gezeirah Shavah.

I think that on the basis of what I wrote above we can now understand why Tosfos maintains that a Hekesh is just as significant as a Gezeirah Shavah. Because the Mitzvos are stated close to each other in the Torah, this gave Chazal the power to compare the Dinim, and it is just as good as the tradition that Chazal had to compare the same two words in different places through a Gezeirah Shavah.

Kol Tuv,

Dovid Bloom