Why Can't People Be Upfront If Something Is Bothering Them?

Say you've become friends with someone and you've conversed for several months, learning about each other, sharing life experiences, you have a lot in common, you laugh and have great long conversations practically every day. Then suddenly, without explanation that person just "cuts out" with no explanation whatsoever. Why can't people be straightforward enough to say what is on their mind, or what is bothering them? This is a male/female friendship btw, and seemed to be a very good one. Do people ever realize how much hurt they cause by treating other people this way, or do they even care???