One of my favorite insights is attributed to Vice President Alben Barkley who, after being informed that a particular person was an avowed enemy, replied, "What did I ever do for him?"
Barkley knew one of the oddities of life: some individuals are shockingly ungrateful. I suspect many of these sad souls construct scenarios that conveniently edit out the assistance they received. The reason for such omissions is unclear - there are others who are similarly situated and yet remain grateful - but it happens frequently enough to be a matter of comment.
You might think that the ingratitude is sparked by some heavy-handed behavior on the part of the favor bearer and that after repeated references to the good deed that was done, the recipent of the favor simply explodes. No doubt that happens and yet I've never found it to be the case.
In my life, I can recall individuals who took the time to help me at certain key points. They didn't have to do any of those acts of kindness, but they did and their intervention made a difference. Should I have resented their actions? Did their good intentions in any way diminish me? I think not.
Ingratitude may be beyond explanation. In one case, when asked by a person to write a job recommendation, I gladly did so. I never heard from the man again.