Only children have no siblings so they are used to entertaining themselves & keeping themselves company. They are very happy indulging in solitary activities such as reading, writing, & drawing among other things. They can freely indulge in such activities w/o the constant interruptions of noisome siblings. They are not afraid to being alone & doing things by themselves, in fact, they relish IT! It doesn't bother only children to do things alone in the least. They are happy creating, paving their way & charting their own territory!
By contrast, children w/siblings are used to having others around. They thrive on the contract interruptions & lack of privacy that siblings provide. As a result of having siblings constantly around, they are very seldom, if ever, alone. They find being alone....quite strange, even aberrant. They must constantly have people around them, if only for a moment. They are, in fact, afraid of being alone for that unsettles, even discombobulate them. The average child w/siblings are needy in that they depend upon others for their emotional, mental, psychological, & even psychic sustenance.