The quality or state of being ardent; passionate intensity, eagerness, or fervent enthusiasm.
From ardent plus the abstract noun suffix -ness, which converts adjectives into nouns describing their qualities.
English loves the -ness suffix for turning adjectives into abstract qualities, and ardentness shows how we can capture 'the quality of being on fire with passion' in a single word.
Complete word intelligence in one call. Free tier — 50 lookups/day.