Ineed, stubborness can just be that: sometimes they just have a different agenda. But what makes them decide they don't want to do... whatever? Lack of motivation/positive reinforcement or some negative feeling about the issue. The balance can be tipped by bribery, greater fear/pain/punishment than would result if the activity went ahead, or removal of the negative feelings by understanding or making the activity less difficult to comprehend.
I personally don't believe horses don't do things simply to be pigheaded.
yes, i agree. in our case our pony associated the trailer with being ridden, usually riding lessons at a yard down the road. he was ridden kindly, in no fear or pain, sound in all ways, with professional instructor - or sometimes just hacking in the forest - but he would quite honestly rather stay at home in the field.
i could de-sensitise him to loading by regularly loading him and not taking him to a lesson, and he would load ok, but if i reinforced his "negative theory" by expecting him to work after being loaded, he would be tricky to load again.
he was always a bit tricky to catch without food aswell, if he thought work was in order. he was just happy being a pony and not career minded.
on a similar note, i sold a 2 yr old pony to a family who wanted a quiet family plod. the pony is question was always mentally going to be a plod, the happiest kindest pony, happy to bumble anywhere and everywhere - now this family have progressed to showing and are wanting this pony to be jumping and performing actively on the bit etc and the pony really doesnt want to and is bucking people off now when he is asked to go into 4th gear. of course he is physically capable of being active but in his heart he just loved ambling along at his own pace, and resents being worked harder than he thinks fit. he is fit and sound, just doesnt enjoy the harder work.
some breeds or individual ponies find normal work much easier and enjoyable than others, and are capable of thinking for themselves.
