Disclaimer: Yeah, I don't own the Sanctus, angels or seraphs
Few angels like humans. Humans are favoured by their creator in a way that some angels begrudge. Not every angel has come to terms with humanity's capacity for free will. Like an elder child usurped from a privileged position, some angels even go so far as to resent humans.
It has been argued that this is the reason for the fall of some of the ambivalent angels during the rebellion. It is a poor leader that cannot take advantage of simmering resentment to curry followers.
In contrast to this, the four seraphim who blaze with holy zeal and stand around their creator's throne, guarding and praising continually and eternally, feel no such resentment. For the humans, in their ingenuity, have given the seraphim a great gift indeed.
The duty of a seraph is to praise their creator with the Sanctus, a repeating mantra of:
'Sanctus, sanctus, sanctus dominus dues sabaoth. Pleni sunt coeli et terra gloria tua. Hosanna in excelsis.'
Heaven is not a place that inspires creativity or art. Perfection inspires nothing but the maintenance of the status quo. After all, if this is perfection, what possible improvement can be made? What reason would there be for innovation? What few angels manage to grasp is that the free will that fed their jealously is by definition an absence of perfection.
The concept of eternity is beyond human conception and likewise the concept of boredom is foreign to a seraph. But the seraphim do not deny the relief they feel as strains of untold variety spring from the minds of humans like Bach, Schubert, Hyden and Verdi. In the absence of perfection, with free will and the necessary suffering it entails, art may be created. Each great composer has their own version of the Sanctus. The feelings evoked and the message given changes wildly despite the identical foundation. The seraphim eternally praise as their nature dictates, but it is humanity that finds the joy in it.
So from music set to a refined string quartet, to the uplifting crash designed for a full symphony, the seraphim praise, with the full might of human creativity spread before them. For this the seraphim appreciate humanity, for even the brightest zeal is cold without the warmth of joy.