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?

Feeling a little calmer?” she asked, leaning against the desk again.Dax snorted. “Mellow as anything. I’m ready to curl up and take a nap now. Don’t mind if I purr.