What piece of music never fails to make you smile or feel uplifted, and why?

So many, but one that since childhood I always go back to is Andras Schiff playing Bach's piano concerto in Dm.

https://www.youtube.com/watch?v=qXQ2yscJ4Mk

Khachaturian’s masquerade suite - just the most lighthearted music I know

Ravels piano concerto for left hand and Adagio for strings.

Last movement of the Brahms Symphony No. 2

Last movement of the Chopin Sonata in b minor

Last page of the Franck Choral in a minor for organ

How is one to choose, there are so many good ones!

For pure simple joy I'd say Chopin Nocturne Op. 9 No. 2, for cheerfulness and brilliance Chopin Etude Op. 10 No. 5, and to add a few heaping tablespoons of excitement and exhilaration Rachmaninoff's Rhapsody on a Theme of Paganini–to name a few, there are too many others!

Mozart no. 16 in c major. Happy happy. 

Oddly enough, it's Schubert's Impromptu in Ab Opus 90 No. 4. I remember listening to this for the first time as a  teenager and thinking this is a very odd 1-4-5 piece. Yet, I was totally drawn to it.  I don't even remember what other pieces were on the album. This is the one that got my attention.  I've  performed it many times probably more than any other piece I've had the opportunity to perform.  There is just something about it... Always makes me smile.

The Schubert Fantasy in F minor (4 hands) has always brought my mood up to a higher level - it's forward motion, the gorgeous melodic themes, it's overall structure - always pushing forward and always moving in a direction of pure bliss for me.