Opposite for me, expressing negative emotions is something I find a hell of a lot tougher than expressing positive ones. ENTPs are notoriously optimistic though, maybe that's the difference there, I don't want the negative emotions to begin with and tend to consider them weaknesses as such I don't want to freely expose to others what I consider flaws. Expressing my deeper personal feelings towards other people is the exception, I tend to suck at that and stick by the whole ‘actions speak louder than words’ deal.
There goes my theory. Actually, I should have thought of that before, since I have an ENTP colleague who rarely if ever expresses negative emotions.