Reason is a servant of the passions. It is one of those "childish things" that Paul tells you not to play with when you get older.

A person does not wake in the morning and say, "I'm going to ponder the various circumstances of my life, and determine in a rational and logical fashion whether I should be grateful or resentful." A person wakes up feeling resentful or grateful, and comes up with justifications for their bias. With the sheer amount of data in a human life, there is a lot of room for "curve-fitting", and thus you get resentful billionaires and ridiculously happy people in slums...

I can understand why a political operator might find a ridiculously happy slum-dweller undesireable, particularly if the politician's social status depends on mobilizing the resentments of the poor, but which of us slum-dwellers wouldn't choose gratitude?