Originally Posted by mr. tegu
SSRIs do not really mask any emotions. Serotonin, and decreased levels of it, has been found to be one of the factors associated with people developing full blown major depressive disorder as well as other conditions. SSRIs allow for increased levels of serotonin in the brain, basically putting the levels back to levels of normal functioning people. So they don't mask the emotions, they minimize the effects of negative emotions that people have.
Instead of feeling completely worthless or guilty as a result of something negative happening to them or performing inadequately (which they will typically ruminate about further worsening the effect of the event), they have a better chance to think rationally about the circumstance and overcome the difficulties. This allows them to think more clearly and allows for more effective therapy. Therapy being the key factor because just taking the medications without therapy is pretty worthless.
Except that SSRIs don't actually work that way for everyone. Prozac, for instance, can leave you disconnected from your emotions and almost zombie-like.