There are somewhere between 700 and 900 different types of Salvia or sage, lots of differences between different plants in that genus. I did some digging on clary sage recently and bought a small bottle of the essential oil.
It's traditional uses in aromatherapy include promoting mental clarity, opposing anxiety and depression, an aphrodisiac, relaxing muscles and reducing spasms, bringing on menstruation (among other things).
This article found that it acts on the dopamine receptors, which are associated with reward/pleasure - that could explain some of the effects on mood.
http://www.ncbi.nlm.nih.gov/pubmed/20441789 In my experience, it's aroma starts out a bit harsh, but it soon becomes sweet. It reminds me of frankincense.