Zoopsia59: I think an important difference between non-musicians and musicians learning to dance is that musicians are used to expressing or interpreting the music directly while dancers express the music they hear. A musician’s job is to take the notes on the page and create emotional content. A dancer hears this in the music and moves his/her body accordingly. Musicians have not (in general) been trained to respond to music by moving, dancing. I can hear what’s happening in a tango piece but have not developed (YET) a way to physically respond, within the tango dance framework. It is as though I hear the music and can enjoy it, but my body is tone-deaf.
I'm not so sure your body is any more tone deaf than the average person. However, the average person who takes a dance class is often NOT the "average person". They are more likely to have natural dance ability than the "average person". That's why they gravitate towards dance classes in the first place.
Sure, not EVERYONE who goes to social dance classes is naturally skilled or gifted, but the percentage of people who are would obviously be greater in a dance class than the general population. I'm not naturally gifted at math (in fact, I totally suck at math) so I'm not all that motivated to take math classes or develop hobbies that require advanced mathematics skills.
I think that musicians have high expectations of themselves because they assume (perhaps subconsciously) that knowledge/talent at music will translate into ability of movement to music.
But they really are two very different things, and an unrealistic expectation of oneself actually impedes progress.. maybe that's why musicians seem to struggle disproportionately in dance classes? (IME)