Different people, different comfort levels
We don't open up the same way to some people as we do others. You might ask, "Does this mean we care more about the ones we share private things with?
Not necessarily. A bevy of factors can be in play, from how long you've known them to their personality and profession.
For example, if your cousin were a psychologist, there's a good chance you might turn to her more often than others when you find yourself in a mental rut.
You might also open up more to those who have shown themselves to be wiser or better listeners.
While my closest friends know me to be fun and silly, I give off a markedly different impression in the workplace, where I tend to be reserved and professional.
Going back to my days in grade school, I've never liked mixing business with pleasure.
Though my coworkers and I get along well, we keep a certain distance from each other, and I prefer it that way. They already know that I'm not the guy you'd probably want to share a bawdy joke with, but the go-to person for all things politics, history, and so forth.
In essence, people assess others' personalities and respond accordingly.
Even age and gender can play a role. Some people feel comfortable talking about certain things (e.g., dating, menstruation) with people of the same sex and/or in a similar stage of life.
In the end, it all boils down to trust and how much you feel you can relate to the other person on said topic. I probably wouldn't talk baseball with a guy who doesn't know the first thing about the sport, or politics with a person who's never voted in her life.
We all talk to certain people about certain things, and that's normal. As we grow more confortable with someone over time, we may begin to let our guard down little by little, resulting in a very close friendship.
However, we should never force ourselves to come out of our comfort zone for any given person unless we feel comfortable doing so.