The affix kyaṅ in the sense of 'feeling', comes after the words sukha 'pleasure' etc. when the pleasure etc. belong to the agent as feels thereof.,
Thus सुखायते ' he feels pleasure'; दुःखायते 'he feels pain'.
Why do we say 'when the feeling belongs to the agent'? Observe सुखं वेदयति प्रसाधको देवदत्तस्य 'the valet knows the happiness of his master Devadatta'. Here the agent viz. a valet, is not the percipient of the 'happiness', which belongs to his master; hence there is no affixing.
सुखादिः।
सुख, दुःख, तृप्त, गहन, कृच्छ्र, अस्, अलीक, प्रतीप, करुण, कृपण, सोट ।,