At one time, "only children" were considered an oddity. And the parents were viewed as somewhat cowardly, stopping after only one child. Or with pity, that the mother must have some serious "internal problems" and couldn't have any more children. That was in the middle of the last century, when the average family consisted of a couple kids, who cleaned up nicely for straight-backed and smiley photographs.
But now the thinking is more balanced. Children in multiple-child families have many advantages, and learn quickly how to "manage" their siblings, while getting enough attention from Mom and Dad. But only children can truly have the best of all worlds.
First of all, they are the sole focus of both parents. This means that they spent a lot of time with Mom and Dad when little, and were probably displaying verbal and even reading skills beyond their age by five years old. They also don't have to share resources with siblings, so only children frequently get the best clothes, go to the best schools, have expensive toys and electronics, and their choice of summer camps and after-school activities.
Without siblings, the only child quickly learns to do for him/herself, which starts and nurtures his independent nature. No older brother to unplug the bathroom drain. No younger sister to steal your cookies. No hand-me-down clothes, or delays of wanted items or services, because "I promised Johnny first." So the only child learns to be independent very young, which in turn gives him/her a sense of empowerment and confidence, that they can do anything if they put their mind to it.
School friends, cousins, Play Dates and other kids can round out the only child's social interactions, and teach the only child the necessary social skils of sharing, politeness and kindness toward others. So the well-raised only-child will be able to make friends, but still be independent enough to forge his/her own paths in Life.
Only children are also the sole repository of parental expectations. Problems can arise if he/she is "expected" to follow in Mom or Dad's profession, or take over the family business. But otherwise, only children learn young, from cues subtle and not so subtle, that they are expected to "make something" of themselves. This helps focus the only child's confidence and empowerment with parental expectations. Only children tend to grow up fast, and display a maturity beyond their years. Sometimes an only child will even talk like an adult at a very young age, since the parents talk and even confide in him/her, as if he was an equal.
Grown only children can be ambitious and driven to succeed. Whereas children from multiple-child familes can grow up to be outstanding managers, only children many times lean toward fields of more individual accomplishment, where they can proceed at their own independent pace.
Another interesting tidbit, is that only children tend to marry other only children. They already have a built-in understanding of each other's background, and can agree on and be enthusiastic about the same things. They are the independent "Let's Go!" types, used to getting what they most desire, and not accustomed to budgetary or other constraints.