Leaving it in the past - issues in coaching

I'm very clear as a coach that I am not a counsellor or therapist. I don't have permission, training or  expertise to go delving about in someone's past. However I'm equally aware that sometimes the roots of present day issues brought into coaching conversations lie in the past.

Take Peter as an example (obviously all specifics are changed!). He is way too nice for his own good - he ends up taking on too much work for others and usually puts his own needs last. He's come to coaching to work on his assertion skills. He's also very aware that this stems from his difficult childhood where pleasing his parents was his default strategy. Knowing this however doesn't help him show up differently at work. He sighs and says something along the lines of 'That is who I am...'.

This is a conundrum for a coach. To what extent is a difficult past a rationalisation for not talking responsibility in the present? Do you take the conversation into discussion of the past in an attempt to undo past wrongs? Or is the work more about acceptance and looking to the future?  To my mind the only legitimate choice is to help the coachee recognise the impact of past experience on the present and help them work out if they want different choices for the future. Do they want keep re-reading the last chapter or start writing the next?