Rabbi Yehuda Amital zt"l suggested that the Sages interpreted Yitzchak’s blessing as referring to God’s consistent but independent acts of giving, as opposed to a constant flow of blessing. Yitzchak wanted his descendants not to enjoy the security of endless resources, of a continuous wellspring of blessing, but rather to turn to the Almighty regularly, each day anew, to ask for a new “giving.” The greatest blessing and gift is not for the Almighty to provide us with a secure, permanent source of sustenance, but rather for us to call out to him each day, and for Him to then answer our prayers, each time anew.