Sniffing happens when a “rabbit brings its nose close to a particular object for a variable period and it is usually accompanied by an increased respiratory rate and an increase in the frequency or amplitude of nostril movements.” 
This one of the body languages which you need to keenly observe to know the message that your pet is putting across.
Why rabbits sniff?
There are several reasons why this pet may sniff you, other rabbits, pets, objects, feces, urine and so on. Some of the common reasons include the following:
Investigating a new environment
These pets are curious creatures and they often explore the environment around them and familiarize themselves with it. When doing so, they will often nudge and sniff everything around them. Sometimes, they might sniff things and nibble them.
For instance, if they feel a scent of a predator around them, you will see them smelling everything including you if you have the scent of their various predators.
It's talking to you
Sometimes, this body language could just be a means of talking to you. Sometimes, bunnies do this to each other followed by bumping each other’s nose when greeting each other.
Agitated or displeased
Sniffing loudly could be a sign of displeasure or agitation and it might be followed by thumping, lunging and biting or nipping if you continue annoying him or her. For instance, this may occur as a “mild protest exhibited by some rabbits when they are picked up” especially the females.
This may also happen as you try to clip their nails. Your rabbit is trying to tell you ‘back-off’ or ‘leave me alone’.
Sometimes a bunny will sniff various things he or she had previously chinned including fecal pellets they had used in marking their territory or even urine he or she had sprayed. They just want to know if someone has invaded their territory.
Bucks are more territorial and tend to spray urine, chin things as well as scatter some territorial feces to help them mark their territory. From time to time, you will see them sniffing both what they had marked and those they have not yet marked.
Sometimes, this behavior could indicate excitement. For instance, “if your bunny has wide-eyes that are paired with sniffing and straight-up ears, it means excitement.” 
It has been exercising
Loud sniffing may follow immediately after your furry critter has been hopping and dancing around you. They are only trying to catch their breath.
From the above scenarios, to be able to tell why your furry friend is showing this behavior, you need to look at the various things that may accompany this body language. This will help avoid misinterpretation since each bunny might have slightly different ways of sending messages.