The Cohanim (Priests), were sanctified by heaven with an extra level holiness and extra commandments. It is therefore fitting for them to keep a higher level of holiness and to behave with greater piety, as the verse says “For the lips of a Cohein will keep knowledge, and they will seek Torah from his mouth”. It is especially incumbent on them to be extra careful with their special commandments, for example the commandment to bless Yisrael, which contains three positive commandments, and the one who blesses receives blessing from Hashem, as it says “And I will bless them”. They also need to be careful not to become impure with the impurity caused by a dead body, and not to marry certain women. Everyone must honor and respect the Cohanim, and not use them, unless he is payed for his work.