A. Their level of abstract reasoning is insufficient to grasp the intent and thus think it stupid,
B. They mistakenly identify with the concept through over-generalization of the underlying premise and thus think the originator vain for assuming he or she was unique in that regard.
Of course, this is just my opinion.
Next thought, in my non sequitorial fashion...
I think that hymnals should be printed with emoticons to help illustrate what kind of mood the composer and author were trying to convey.
"All people that on earth do dwell,
Sing to the Lord with cheerful voice. =)
Him serve with mirth, =D his praise forth tell;
Come ye before him and rejoice. =D"
Or something along those lines...