There were many times that I have experienced dreams where my two Grandmas, my Uncle, and my friend who died at an early age had appeared to me in a dream. They had a message to tell me and that was that they were ok now and that they were safe on the other side. They do communicate, especially to help people to move on during their grieving process. Some of the dead try to comfort those who are experiencing grief or frustration of their lose. The dead, I believe are happier because they are in a better place. That is just one way they appear in dreams.
Another, is that they may tells us an important message that they couldn't tell us when they were living that is essential for us to know. They are gone in this life, but they still exist in another realm. The spirit never dies. Except it may be attached to the body and may be grieving itself that it doesn't want to let go of all that it had in this life.
The Dead may warn us of future events that may effect us and the world. They gives us these insight because it may be helpful to the evolution of their world and the other realm as well. The dead communicate in many ways. Dr. Michael Newton writes in the Journey of the Soul and the Destiny of the Soul about the life between lives. In it, Dr. Newton discusses the cases studies of people during hypnosis. They regressed not in past lives, but they told of experiences of life between lives. In the first section, he discussed how the dead communicated with the living. He mentioned through usage of songs, TV shows, objects and even dreams that they communicated a certain message to the living. They communicated with their friends, family and spouses through different spiritual techniques as a spirit to let them know that they are OK and that their spirit lives on.
Since we are living in this lifetime we feel as if life is all in this existance. Little do we know there is more to life than we know. When we are speaking, I believe, we are intact with the spiritual world. And we are aware of what goes on, except when we wake we forget everything on the other side because we are working on our spirit as a human. When we die there is no such thing as time. Spirits can appear to us in dreams or any other form they want. When people see ghosts or any other spiritual object that may move and they haven't seen it before. It may scare the hell out of them, but it is possible. With dreams, our beloved dead, now spirits in the spiritual realm, we'll communicate with us to let us know that they are there. And we believe that they are still here in our realm, until we wake up and realize that our beloved is still here, but in the spiritual world now.