In MBTI theory, as an ENTP my 'feeling' is extroverted, and I think it is true that I do monitor other people's values and reactions in order to work out what I believe. So I am chewing over those comments and considering whether I was wrong.
I think moral values are social, and to some extent I do amend my values if a point seems important to other people. An example would be gay marriage, where at one time my idea was 'we should all reject marriage, whether we are gay or straight' but it has changed to 'let people choose what makes them happy'. I'm a humanist, that is, my moral values are largely determined by what causes happiness or suffering - there are many other legitimate frames of reference though.
There are dangers in both introverted and extroverted 'feeling'. I think for a person with extroverted feeling the danger is that one becomes a social conformist or a moral butterfly, blown here and there by majority opinion. I think for a person of introverted moral judgement, the danger is the opposite - inflexibility for example. I'm absolutely not saying these are inevitable faults, of course.
People with strong religious beliefs often ask humanists 'where do you get your morality from?' if not from the precepts of a religion. Who knows? I literally don't know. My guess is morality doesn't come from one source, but from a composite of biological and intellectual tendencies, all pulling in different directions.
Arguments and contrary opinions welcome as ever.