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So was Sam really in the past or was he mad? Here are my own conclusions.
For me Sam had been transported back in time after his accident. When he returned to the future he realised that he belonged, and could do more good, in 1973. Sam wasn't really living in 2006, he was just existing, going through the motions. In 2006 Sam was a very small cog in a very large machine, with very little influence on the world he inhabited. He just didn't feel he belonged. Realising this he committs suicide and returns to 1973, a place where he feels alive.
This view of the ending can be disputed and this is what I mean by 'good' drama. You could argue that Sam was in fact from 1973 and because of the childhood trauma, revealed in the final episode, he had created the vision of the future to cope with the loss of his parents. 2006 was all in his imagination, an illusion reborn from a recent accident. Going back to 'his' 2006 Sam was having a breakdown. Making a conscious decision to stay in 1973 he finally saved himself and his sanity.
These are only two interpretations of the ending and I'm sure others have come up with many more.
And then of course there's the ending I would have written.
Sam discovers Gene Hunt was the person who ran him over in 2006 and to get home Sam has to destroy Gene Hunt in 1973. However, by doing so he has to destroy the rest of the team, people he considers friends. Sam realises he can't do this and saves the team and Gene Hunt, sacrificing his own life for the sake of the others.
What did the finale of Life On Mars mean to you?