That's the way children associate themselves with objects. First the mouth, then the mouth again. They are trying to "relate" to an object or thing. Just like the hands go into the mouth and so on.
To "familiarize" themselves to this new and exciting world they've come into. Actually, it's innate to say the least. It can't be stopped. It's learned and innate. The bottle, the pacifier just about everything goes straight to mouth. It goes back to the days of cavemen. Actually, scientists are claiming that "kissing" goes back to the "nurturing" stimuli that women learned by feeding their young via mouth from first chewing certain food(s) and then placing the half dissolved particles of food in the babies mouths to eat. It's all innate, really. The mouth is basically where most "stimuli" reside, therefore, into the mouth it goes.
What better way to get to know something then by sticking it in your mouth? The taste, texture and even smell (mouth is close to the nose) are all absorbed by one simple action.