Suck is the first thing any mammal does next to breathe after it has been born. If a creature cannot suck it's mother's milk it will die {unless it is an infant human}. In which case the medical profession won't allow it to die but will supplant the lack of sucking with IV drips and other artificial life prolonging methods.
As we grow up some get over the need to suck but most of us keep the desire to suck and hence the continuing success of oral sex, nipples, icecreams, lollipops, kissing,toothpicks, bananas, fingers, bubblegum, drinking straws in bottles of coke... well the list of things we put in our mouths and suck on is endless. Since sucking is so cool and pleasurable - how come it is associated with 'bad' things, with negative things? It's a mystery to me.