But yet, this is exactly what you have to resist doing unless you want them to think that the World Revolves Around Them. We already have to contend with this and she's only four. I think when the child is the only child they are more apt to get the wrong idea by default at any early age. They have to learn about sharing and the parents have to teach them that, which is a difficult topic anyway in the best of set ups with multiple brothers and sisters, let alone when they are the only child in the house.
Having brothers and/or sisters a given child is going to learn about the law of 'common property' whether they like it or not. They will also have to learn--visiting it daily--about the principle of taking turns which, when they are the only one, isn't really forced on them to a true degree, i.e. peer to peer, until they are in Pre-K most of the week, every week. This, to me, certainly isn't a reason to have more kids than you want to, but it is a real world challenge when you have only one, and perhaps a little extra work--no not perhaps. It is a little extra work for his or her parents to teach that principle in a loving and smooth way from day to day through the toddling years.