How do protic solvents decrease nucleophilicity?
The protons released by the protic solvent surround the electron-rich (aka Nucleophilic) species and attract electron densities towards themselves. As a result, the nucleophile becomes satisfied i.e. its need to donate its electrons to another electron deficient species (Electrophile) decreases. It becomes less nucleophilic.
Ex: H2O is a protic solvent.