Sense of Humour
If you lack one, you should never have joined. Also worth noting is 'sense of humour failure' - the point where lack of sleep, rations, good weather, perceived incompetence of juniors/seniors all conspire to ruin what has been, up to now, a good day.
The problem with a sense of humour is that everyone's different. What one may find keck-wettingly hilarious, e.g. skiffing, might not necessarily be to others' tastes. In fact I can positively guarantee it. Try it with your bank manager and see him erupt with laughter before increasing your overdraft. Likewise, an impromptu game of speckles is under-appreciated at the WI gala... fucking lightweights.
The other problem with humour is its bastard lovechild: squaddie humour. Squaddie humour is understood little outside of the closed world of service life. It does not travel well - mainly due to its sick, perverted and twisted content.
Squaddie humour is not just black, it's blacker than a black thing in a darkened room smeared in boot polish and covered in Harry Maskers. A squaddie will drag humour from the depths of despair and turn a hopeless tragedy in to a thing of great mirth. Try it next time your in the Maternity Ward. 'Oi sweetie! What's six inches long and makes women scream? Stillbirth!' Ho ho.