A lot going on in this thread and I have some thoughts on specific comments, but for now I'd like to be clear about what I mean when I use tranquility or ataraxia.
I don't mean some mystical state or some "special" state or some woo-woo state.
I do mean simply a clear-headed, calm mind unruffled by anxiety or fear.
A person can have that state if they are relaxing, if they are engaged in action, even if they're on the battlefield. It means someone isn't freaking out. It means they approach decisions clearly, decisively, with no equivocation or regret.
Does that state arise naturally? Yes.
Does it take practice to achieve and maintain that state? Absolutely.
Is it better to have that state as a foundation from which to confront the "slings and arrows" of daily life than other states? Yes indeed, in my opinion.
In the end, I don't think one can truly be happy, be filled with well-being, or experience satisfaction unless you're working towards having that calm baseline to work from.