I was recently prescribed 10 mg nortriptyline; I've not been able to tolerate most SSRIs (except for zoloft, and only at a low dose that still made me feel sleepless/jittery). My pdoc's rationale is that I metabolize drugs much more slowly, and Pamelor in particular is extensively metabolized in the liver, so that is why we start at the low dose.
A secondary benefit is that part of my anxiety manifests itself with muscle aches, headaches, and irritable bowel type symptoms, which low dose pamelor is often prescribed to treat. I strongly recommend listening to your doctor and asking him questions; there are no strict rules to psychoactive medications. We each react differently... some people require very high doses, and some people need very low doses. The "therapeutic dose" of pamelor is actually closer to 75-100 mgs, with 150 mg being the recommended max. Pamelor is also one of the few meds where you can take a blood test for the therapeutic level in your blood.