Yes, I've noticed that too, but it's hard to say what it is: is it a simple placebo effect, or is it the miser in me saying 'you spent $1.20 on modafinil for today, and dammit you'd better get >1.20 out of it!', or is it the reduction of tiredness, or the sense of lots of time in front of one (I think of Lin Yutang's quote: "A man who has to be punctually at a certain place at five o'clock has the whole afternoon from one to five ruined for him already.")?

Or something entirely else, like that one notices the drop in motivation only when stopping modafinil, and this drop might be due just to recovery from usage? (A slow depletion of dopamine, eg.)

If it's this last suggestion, then the motivation effect is just a modest version of the amphetamine motivation-then-crash - but what makes modafinil most interesting is that it by and large seems like a 'free lunch', and those are so rare in biology/pharmaceuticals.