Rashi interprets the pasuk above to mean that whoever blesses his sons will bless them with their (Efraim and Menashe’s) blessing. Indeed, the standard blessing of father to son is, “Hashem shall make you as Efraim and as Menashe.” What was unique about these two brothers that so endeared them to Yaakov? Why do they, from amongst all the tribes, stand conspicuous as the paradigm for blessing?
Horav Eliezer Sorotzkin, Shlita, suggests the following reason. Although the shevatim, tribes, were unique in their yiraas shomayim, fear of Heaven, they were regrettably hampered by strife and discord. The brothers’ jealousy of Yosef caused tragedy and misfortune. When Yaakov conferred blessing upon Efraim, the younger brother, Menashe demonstrated no envy. Only Yosef took issue with Yaakov’s blessing the younger one prior to the older one. Indeed, when Yosef interjected on behalf of Menashe, Efraim did not take offense. When Yaakov noticed the harmonious relationship exhibited by these two brothers, the mutual respect and esteem in which they held one another, he chose them for blessing. First and foremost in the heart and mind of every parent is that their children maintain relationships in which they are each devoted to the other.
