A couple of years ago I had a job that required me to arrive at 5 a.m., as I did not own a car at the time. So my girlfriend used to drag herself out of bed every morning at 4:30 and drive my sorry ass down there. Though I bought a vehicle shortly thereafter, the memory of what my wife (incredibly, she agreed to marry me after this) did for me over the course of those months continues to haunt me.
Of course, my father did far more when I was an infant. Fortunately, evolution has designed us to forget everything before the age of six so we can enter our teens and 20s completely free of any sense of obligation to our parents, and therefore devote the time we would otherwise have used to call them to replaying World 4-1 of Super Mario Land in the hopes of getting our record time below 160 seconds.
And then one day you're mopping excrement off the nether end of your own child and thinking, "Man, this is a lousy job. I hope when this kid is my age he appreciates all I've done for him as much as I appreciate all that my…uh…" And then there it is: the specter of used-to-get-up-at-three-in-the-morning-to-make-sure-I-hadn't-kicked-off-my-blankets takes up residence in the back of your head and insistently pokes you in the aft-brain whenever you interact with your own father from that day forward.
If often think along these lines. It start with something like 'The statute of limitations on what my twins owe me (and Christine) will never expire.' Not in some kind of miserly tit-for-tat way, but just reflecting on the nature of our relationship. But then I invariably think 'Of course, that's not how I treat my parents.' And I can't make myself. I still think stuff like 'They can call me this week.' But I'll cycle back tomorrow to 'Wil and Elizabeth owe me for the rest of our lives.'
To touch on Matthew's phrasing, while the generosity of others haunts me, it seldom really changes me. Gratitude is rarely felt deeply enough to impact behavior long term, at least in my life.
In that vein, Alex has a post about the birth of their second child and the Fatherhood of God (and I had a quasi-objection in the comments).