What are the causes of communication gap?

First of all there are different ways to communicate. There is texting, phone and face to face communication among many others. Let me make this clear: I am addressing face to face communication. When we are talking about 'lack of communication' then we must address most of all direct encounters. The primary cause for the lack of face to face comminication is the fear of angry exchanges. We all tend to back off from a fight. But this is exactly why this fear is the main cause of a lack of communication. We are afraid to cause anger, for anger will disturb what might be called our "need to find approval." But often it is anger, a free exchange of our position on a problem, that is the essential ingrediant of problem solving. And relationships can only exist if we get into the habit of consistently resolving differences. There is a whole section about problem solving called 'The Tools' in "Marriages, Shack-Ups and Other Disasters" - see web link below. I would suggest that you check out 'The Framing of Anger' in particular.