I think....
People do not discuss things they are not comfortable discussing.
People are often uncomfortable discussing things because of what others may think of them; they fear rejection of their intimate thoughts and pains.
There are a lot of reasons that a person may not want to talk about something, but the most important thing is, they shouldn't have to. If you would like someone to talk about something, you have the right to voice that but every person has the right to say no. Just because you think a thought or idea would be useful or helpful to others, or that it hurts them deeply and it would help them to share, does not make it so.
Every individual is responsible for handling things in a manner that they feel fits with their inner path, and I think that if a person doesn't want to share something difficult, I can work that into my inner path without judgment unto them or trying to theorize why this is the case.