You are always so cheerful! It is contagious.
'Allegro' = al-LE-gro, stress on second syllable. 'Contagioso' = con-ta-GIO-so.
Use this to compliment someone's positive energy at a social gathering. 'Allegro/a' (cheerful) is a lovely character compliment. 'È contagioso' (it is contagious) adds a metaphor that acknowledges the social impact of their positivity.
'Sempre così' (always so) can imply familiarity — you have met them before, or you are inferring their character quickly. The metaphor of 'contagioso' is very Italian — joy as something that spreads from person to person like a pleasant virus.
Stai di buon umore stasera!
You are in a good mood tonight!
More specific to the current evening
La tua positività si sente.
Your positivity can be felt.
Metaphysical — positivity as a palpable quality
Fai ridere tutto il gruppo!
You make the whole group laugh!
Social impact of their cheerfulness
Italian social gatherings prize 'la leggerezza' (lightness, carefree joy) — the ability to be present, joyful, and entertaining. The person who makes everyone laugh ('quello che fa ridere tutti') is a treasured social figure. Italian comedy traditions from Commedia dell'Arte to modern stand-up all reflect this deep cultural appreciation for joyful wit.