Dominate Person new saving throw conditions

[Bastok]

I had recently used Dominate Person on a giant and he failed his ST. Due to our horses (but not our cart) getting caught in an explosion, I decided that I would just get our giant to pull the thing. As soon as I did, our GM said that the giant gets a new ST with the +2 bonus because he "doesn't want to pull the cart". By that logic, we could go on to say that no creature wants to be under the effects of Dominate Person and as a result, would get an ST every round, even out of combat. My question is; What would ultimately be considered against someone's nature?

[DonDuckie]

My opinion is: something the creature would never do under "normal" circumstances.

A lawful person wouldn't steal from his liege lord.
A good person wouldn't murder an innocent.
A rich, greedy person wouldn't give away his wealth.
An honorbound assassin wouldn't give up his client.

"A giant doesn't want to pull something" doesn't qualify, because he would probably do it if it was his own cart, or he was paid to do it.

It has to be something that goes against the established nature of the individual.

Eg.: If the giant was a former slave pulling carts, and - after escaping - having decided never to do so again and hates carts and being told to pull them... that I would consider against his nature.

In any case(my ruling): the situation that allows the extra save only allows one attempt... if that attempt fails the task is carried out. It's not a new save per round, it's one save per task.

[Bastok]

Thanks a bunch.