I am a person with a high need for socialization. Throughout my life, I have strongly sought to be around people.
Our kids provide that socialization in spades. I find that I need only ten percent of the outside socialization I needed previous to having kids. And when I got married, I needed only half the socialization I needed prior to that.
As our kids grow and learn and can do more, they will take up more and more of our socialization. I think we are close to the point where we will actually seek bits of alone time rather than socialization.
This is why many single friends of people who have kids are disappointed when their married friends disappear. Those single friends still need socialization, but the married friends do not. The married friends with kids actually often are desperate for alone time.
Plus most people with kids who put effort into their kids would rather spend time with their kids than anyone else. Everyone else, even parents or grandparents or best friends just start to pale by comparison. I guess this is a biological drive to maximize the survival of your genes.