Everyone knows that audience members at concerts are never happy.
The etiquette is to sit quietly in the dark. Suppress all emotional response and bodily noise. Looking happy or as if you are enjoying yourself is resolutely low rent... Schubert and Brahms were miserable, and you should be, too. The musicians aren’t there to entertain you. Conductors are there to flap their arms, wiggle their bottoms, and get paid a lot. Just look around you, right?