The gratitude of most men is but a secret desire of receiving greater benefits.

A good deal of what looks like thanks is an investment in the next favour. It is warmest where the giver can still give more and thins out noticeably once they cannot. Not all gratitude is calculation, plainly, since people do feel genuine thanks. But the calculating kind explains why some of it evaporates the moment a person has nothing further to offer.

Look at who thanked you and who kept in touch after you had nothing left to give them. And on your side, go back to someone who helped you years ago and say so now, when there is nothing further to be got from them.