Shells contains a lot of flavours and compounds.
Boil shells in liquid for about 5 minutes for maximum flavour. Additional times reduces the flavour as it gets aromatized. Also, brown the shells first so that the oil absorbs some of those flavour compounds, and also generates more from the Maillard reaction.
Look for individually frozen shrimp, and ideally the shrimp does not have sodium triphosphate () in them which deteriorates texture.