Why Kufr?
The sixth kind of apostasy is mockery of what Allah sent down or of anything that the Messenger came with, even things from the Sunnahs (of the Prophet, sallallaahu 'alayhe wa sallam) or recommended things like siwaak, trimming the moustache, removing armpit hair, or trimming the nails. If a person makes mockery of any of these things he becomes a disbeliever. The evidence for this is found in the Statement of Allah the Most High:
"And if you were to ask them, they would say: 'Surely we were
only jesting and playing.' Say: 'Was it Allah, His Verses,
or His Messenger you were mocking? Make no mistake,
verily you have disbelieved after your belief.' surah at-tawbah
Even if a person falls into some kind of opposition (to the Sunnah) due to his desires, then surely he still must respect the way of the Messenger (sallallaahu 'alayhe wa sallam), respect his Sunnahs, and respect the narrations.
http://www.bakkah.net/articles/mocking-Islam-disbelief.htm