[Francis] Flynn asserts that immediately after one person performs a favor for another, the recipient of the favor places more value on the favor than does the favor-doer. However, as time passes, the value of the favor decreases in the recipient's eyes, whereas for the favor-doer, it actually increases. Although there are several potential reasons for this discrepancy, one possibility is that, as time goes by, the memory of the favor-doing event gets distorted, and since people have the desire to see themselves in the best possible light, receivers may think they didn't need all that much help at the time, while givers may think they really went out of their way for the receiver.
That is from Robert B. Cialdini's fascinating Yes! 50 Scientifically Proven Ways to be Persuasive. Cialdini's earlier Influence remains one of my favorite social science books. Here is a link to Flynn's paper and related work.