TT #44 · Parashas Va’eschanan · Passuk 7:9
Know therefore that Hashem your God is God, the faithful God who keeps the covenant and the lovingkindness with those who love Him and keep His commandments to a thousand generations.
Torah Temimah points to the comparison of one who serves from love (reward to a thousand generations) versus one who serves from fear (Sotah 31a parallel / baraisa of R’ Shimon ben Elazar).
English: It was taught, R’ Shimon ben Elazar says: Greater is one who acts from love than one who acts from fear, for the former is dependent on a thousand generations and the latter on two thousand (or vice versa according to the reading of the parallel verses).
Diyuk of the differing generational numbers for love versus fear.
Service from love is ranked above service from fear by the differing spans of reward attached to each in the verses.