Because you have to just go with the flow. Your life is not your own, with people coming in and out all the time. You get mellow because you have to.
Though why should we expect age to mellow us? If it isn't life's business to reward merit, why should it be life's business to give us warm, comfortable feelings towards its end? What possible evolutionary purpose could nostalgia serve?
We came out in the midst of the hippie hangover. All this mellow music.
