As an animal owner, it is your responsibility to feed and take care of them. Part of being a good hooman to your pet is knowing ways to communicate with them and knowing why your pet behaves in a particular way.
There are people who know reasons why their dogs smell their private areas, but also some people aren’t yet aware what this behaviour pertains.
The reason behind dogs sniffing behaviour according to a site named Master Bowie,
“is gathering information and saying hello. We have sweat glands, apocrine glands to be precise, which release pheromones that convey all different types of information (eg. age, sex, mood, health). These glands are heavily concentrated in the armpits and genitals.
Well… since they can’t reach the armpits, so crotch it is…”.

Dogs cannot speak so they have their own ways. Your dog is just trying to know you, but if you’re embarrassed with this innate behaviour, you can redirect it by training your dog; giving treats and saying basic commands such as sit, stay, etc.

Their sniffing is equivalent to asking your name, where you live, as well as shaking hands. As non-verbal.