Non-human animals are better able to live in the present moment. This is both a survival mechanism, but also provides a concrete reason why non-human animals do not ruminate upon the past with regret and/or longing. Other animals do, of course, learn lessons from the past. A puppy or kitten traumatized in infancy by a dog, person or even a car, will have imprinted safeguards that keep them forever wary about such threats. However, this is automatic fight of flight response of each unfolding moment and not an on-going, worrying stress for them.
For humans, there is much concern about the past. When traumatized as children, they will have the same automatic fight or flight response as other animals. They will also, however, have an added dose of accumulated stress, worry, concern, fear, anxiety and generally unpleasant stress about their internalized trauma. It can destroy or sabotage relationships. It destroys trust and ability for people to hold high self esteem. It can, and does most of all, take people out of the present moment. Instead of living life, they self-program to worry about regrets, and fears of the past and how to cope with uncertainty and anxiety about the future. People re-live the past and embroider it and its injury potential by playing painful tapes over and over again in a fearful mind.
Some expression of past trauma is healthy. Expressing it, in a controlled and emotionally supported setting is regenerating and healing. It is containing it, hiding it, denying it or simply storing it that is toxic. Like an acrid poison that eats from the inside out, eroding its container, the toxins of past painful emotions can destroy not just the present moment, but the container too.
Finding relationships is the greatest concern for human beings. Psychologists such as Martin Seligman have demonstrated in collaboration with teams of researchers, that happiness is determined by relationship and social interaction. If those social interactions have meaning, pleasure and "flow" ( a sense of being in the moment, not immersed in past loss or future anxiety) human beings master psychological happiness.
Taking it one step farther than human relationships is also extremely healthy. Just as people can learn by observing their pet cat or dog, that living in the moment is rich and fulfilling, they can learn by constantly nurturing relationships among other animals, forests, systems of nature that are regenerative, supportive and trustworthy. Finding appreciation for life itself, (not just dreary human life) is healing in ways that are profound. The healing of the whole earth from human degradation offers people the chance to not only belong, but to be heroes rather than victims of their past.