The first recipe for happiness is: Avoid too lengthy meditation on the past. -Andre Maurois
I suppose the key is “too lengthy” – it seems inevitable to look back now and then. This could be chalked up to human nature – do animals ever look back on the past? – and if it is human nature, there must be a reason for us to feel compelled to look back. Except for a periodic progress check (or maybe an anecdote to help someone), I find little reason to look at the past, let alone dwell there.