Ajay do you have good answer for this. I don't know exactly because babies can't speak. If I don't let baby suck his thumb, he would stsrt crying. He loves to do so.
Well Divyani, I have a project in mind regarding the behavioral characteristics of babies or infants but somehow got stuck at this point. So far whatever data I collected regarding the same says that they enjoy sucking as a separate experience than feeding. They indulge themselves in such a behavior when they are basically tired, bored or in want of comfort or going through hunger.
Besides, it is also a fact that some of them can be comforted by using pacifier...!