I think this advice is all well and good - in fact I think it's often very helpful to take a step back and view your life as an unfolding story, and then ask yourself whether it's unfolding the way you want it to and whether you are playing the role in it that you would like. However, it also occurred to me that the assumption that one is automatically the protagonist of the story seems a very Western, individualistic assumption. Of course we all feel that we should be the captain of our own ship, the center of our own universe, the main character in the drama of our lives - but would everyone feel that way? There are cultures (and time periods in our own past) where it would not have just been taken for granted that we ourselves are the most important players in our own lives. In more communal, collectivist, or family/clan oriented societies there are probably many people who see the narrative of their lives as fulfilling a supporting role in someone else's story - or, even more likely, as a supporting role in the much larger story of the community as a whole. (In other words, it's an ensemble piece.) And what is more, they would be okay with this - people would actually find positive self-worth and identity by living their lives as helpers and supporters of others.
Of course as a product of my own Western culture (and as someone attuned to the injustices of imposed social classes and norms) I'm conflicted about this issue. On the one hand, I can see the danger for abuse. Far too many people - women and minorities especially - have too often been forcibly cast in a supporting role without any choice of their own. Many people who would much more naturally fit into a leading role are suppressed and ignored and told that their only purpose and identity is to serve those with power over them, the true protagonists - their husband, white people, their employer, their owner, etc. They are never given the option of becoming protagonists themselves, and that is just plain wrong.
On the other hand, the reaction to this can sometimes be to insist that everyone should play a leading role, but I'm not sure that is right either. When you have a world full of people who all think that the world revolves around them... well, you can imagine what messy complications that might lead to (or you can just read your daily newspaper). Also, what of the person who genuinely desires a supporting role, but is made to feel guilty for not being more "ambitious"? Do we undermine the happiness of those who are just natural "helpers" by insisting that they assert themselves more? I'm just not sure everyone can or should be a protagonist. What if the purpose of your life really is to play a supporting role in some larger story? And what if that's a good thing?
Perhaps the answer is that in the end we are all just supporting roles in the really big story that God is telling. Some of us might have more prominent parts in the narrative, but the less prominent, less noticed parts are no less important. We all have our roles to play, and the point is not to steal the show and grab all the glory for ourselves, but to help tell the best story possible... all of us, together.