condolence n.

c. 1600, "sympathetic grief, sorrowing with another" (a sense now obsolete); 1610s, "expression of sympathy to one in distress, mourning, etc.," from Late Latin condolens, present participle of  condolere "to suffer with another" (from assimilated form of com "with, together" + dolere "to grieve;" see doleful) + -ence. Often in form condoleance 1600-1800. Condolent, "sympathizing, compassionate," is attested from c. 1500, from Latin condolentem.

Condolences "formal declaration of sympathy" is from 1670s; the reason for it being always plural is unclear, but the earliest references are to expressions from groups of persons ("Foreign Princes addressed their Condolences to him"), so perhaps the habit stuck.