There is a theory that recurrent, treatment-resistant depression (TRD) is more similar to bipolar disorder than to "regular" (unipolar) depression. Because of this, folks with TRD respond better to an antidepressant plus a mood stabilizer. This may be your pdoc's reasoning?

I know that this I the case for me. I have never had any mania, but antidepressants alone are not nearly as helpful as an antidepressant with a mood stabilizer. I resisted trying a mood stabilizer for a long time due to misconceptions, but I greatly improved when I tried it.

Lots of luck,
EJ