A feature of lashon hakodesh (Biblical Hebrew) is that a verb is sometimes followed by "לו", inflected for the person, number, and gender of the subject, without the "לו" indicating an object of the verb. This occurs frequently in Tanach, and also later (e.g. "בא לו אצל רבי דוסא בן הרכינס" in mishnayos Rosh Hashana chapter 2). At some, but not all, of its appearances in chumash, Rashi comments on it.
Note that Gesenius (FWIW) says that this "לו" has no specific meaning itself, but does change the meaning of the sentence slightly: it is there (in his words) "to give emphasis to the significance of the occurrence in question for a particular subject", namely for the subject of the verb (or more precisely for the referent of the subject of the verb).
My first question is: When Rashi comments on the presence of "לו" in chumash, is it:
- because he views such a construction as purely pleonastic (rhetorical) in general (i.e., bearing no connotation whatsoever, not like Gesenius), and wishes to explain its presence in that location, specifically, as not purely pleonastic but, rather, bearing the meaning he mentions; or
- because he views such a construction as purely pleonastic everywhere, and wishes to explain its presence in that location b'derech d'rash; or
- because he views such a construction, in general, as, l'havdil, Gesenius does (that the word is adding emphasis of the significance of the action to its subject) and merely wishes to explain in greater detail how or why, in that location, that act is particularly significant to its subject; or
- for some other reason?
And my second question, which can only really be answered once the first is (i.e., once we know why Rashi is commenting at all) is: Why does Rashi choose to comment on some of these and not on others? For example, he comments at B'reshis 12:1, "לֶךְ לְךָ מֵאַרְצְךָ וּמִמּוֹלַדְתְּךָ וּמִבֵּית אָבִיךָ", but not at D'varim 16:9, "שִׁבְעָה שָׁבֻעֹת תִּסְפָּר לָךְ".