Light and heat
Light fades colour and heat drives off aroma, so spices belong in the dark and cool.
Also: sunlight damage, stove heat, photo-oxidation, storage temperature
Light, especially sunlight, breaks down pigments such as the carotenoids in paprika and chilli and curcumin in turmeric, and speeds oxidation of oils. Heat raises evaporation of volatile compounds. A clear jar on a sunny shelf next to the oven is the worst place. Opaque or dark containers, or a drawer or cupboard, are better. A cool, steady temperature, below about 25 degrees Celsius, slows deterioration, and refrigeration helps chilli and paprika, though condensation risk means containers should be sealed. Spice shops in old bazaars used cloth sacks in cool, dim rooms, for good reason. Keep colourful red spices in particular out of light.