Why do dogs have to smell your guest's crotch?
Since we human are part of petthey see at us no different than them, therefore, they see us as peers – with a certain higher hierarchy that is- that should go through the same identification process as any other of the pack.
In other wordssmell our crotches to identify us. Although it is reasonable to think that the scents exuded in certain areas of our body are more attractive to them than others. Also, the crotch is most often at the same level as their olfactory skills.