I need the loo?
Aside from the stress, comes the boredom. When I am driving/flying I am busy and my body doesn't think about the need for a break in the same way as when I am sitting looking at the back of the seat in front.
Also have you ever seen the pax facilities at the toilets at say LGW? STN? or worst - LHR T2/T3? The gutter outside is the more sanitary option.