Or HaChaim on Deuteronomy 7:9/אור החיים על דברים ז׳:ט׳

1 ולשומרי מצותיו לאלף דור, "and to those who observe His commandments for a thousand generations." Our sages in Sotah 31 say that those who perform the commandments out of fear will be favourably remembered for one thousand generations whereas those who perform the commandments out of love will be favourably remembered for two thousand generations. This is parallel to what is written in the second of the Ten Commandments (Exodus 20,6). I do not believe that the word אלפים in Exodus 20,6 means merely twice one thousand; it may mean as many as 100,000. The reason that the Torah was not more specific is because love cannot be measured so precisely, some people love G'd more than others. As a result, G'd requites some people's love for longer periods than others. What the Torah did postulate was that he who serves G'd out of love, even if his love is minimal, deserves twice as much consideration as he who serves G'd out of fear though out of the most refined kind of reverence.