This related M.Y. answer explains that there is no specific mitzvah to love one's parents and the concept is incorporated in the mitzvah of loving one's fellow.
There is a mitzvah to love the convert, one's fellow, and G-d himself. Why does the Torah not specify loving one's parents, rather than incorporating them into the mitzvah of one of the others (as stated in the linked answer)?
I heard (don't recall source, offhand) that one receives greater reward for performing a mitzvah because it was specifically commanded as a mitzvah than doing it because it is the natural, nice or logical thing to do.
If that principle is true, why shouldn't this mitzvah be explicitly listed so that one can receive greater reward for it?