People in the book avoid deep conversations because it endangers them. Think of all the characters who are involved in these activities--the old woman who doesn't want her books burned, Clarisse, Professor West, all the book people who memorize books for future generations--they are outcasts or disappear altogether because they refuse to be swallowed up by the meaningless lifestyle of sitting around and watching the walls of one's home. Mildred and her friends are more involved in the lives of soap opera characters than they are in one another's. It's easier to learn about false people--safe people--than it is to talk about deeper subjects and controversial topics with real individuals, especially if you are unsure of whom to trust.
Any way you look at it, in Montag's world or ours, to become deeply involved in real conversation and deep feelings with other people is to take a risk. Most people prefer to protect themselves than to make themselves vulnerable.
I'm not sure if your question is asking about the characters of "Fahrenheit 451" or about people in society today. In the book, the people aren't interested in real conversations and deep feelings because they've become too accustomed to a life where they don't have to think about conversing with another person to any depth. Their world has made life such that people occupy their time with mindless activities and real conversation requires thought. People tend to reveal themselves when they talk to other people, either by what they state or even by their reactions to what others state. The government in "Fahrenheit 451" wanted control of the people and it's much easier to control people who don't think, so real conversation was discouraged. Capt. Beatty explains to Montag the day Montag stays home from work, that if people have mindless activities to fill up their time -trivia contests, sports, etc. - then the people are happy. Happy people are much easier to control. The people of "Fahrenheit 451" do not develop deep feelings for other people for the same reason. Deep feelings usually result in extremes - either great joy or great sorrow. This breadth of emotions, especially the great sorry, can cause confusion and sadness among the people. People didn't want to deal with depth of emotion and so limiting one's relationships helped to ease that in Montag's world. Again, the government discouraged deep feelings among the people. They kept people busy as much as possible doing fun things. They put children in schools as early as possible which kept parents from developing deep ties to their kids and then, through learned behaviors, kept children from seeing what deep attachments were like and therefore kept them from forming some of their own.
I'm not a psychologist so I can't give you a definitive answer as to why some people in our society don't conduct real conversations with other people and why they don't develop deep feelings for others. I assume though that the reason some people are like this is because they don't want to think and because they are afraid to have deep feelings for other people out of fear of having deep and painful feelings. I think the reasons are numerous and diverse.
There is clear evidence that children of parents good in their conversation abilities and socil skills also tend to be skilles similarly. But the reason of these is not heredety. Children learn these skills by observing and unconsciously copying their parents.
I have the feeling that it is done through conditioning of our actions from heredity and environment. If our parents were skilled conversationalists, then I believe that we would be disposed to having natural conversational abilities.
Also, from our environment, ever since we started to have more ways to communicate, our skills, it appears, have been dwindling. How many times do you hear about teenagers who didn't actually talk on the phone, just listen to each other breathing? Or if someone just utters guttural sounds of acknowledgement like "eh", or "uh-huh."
I think we are losing our skills for conversing due to the fact that we can almost contact each through far too many ways. We are becoming a "Twittering" society of not more than 140 characters at a time.