Well, it's only because I know I get stressed fairly easily, and am rather highstrung, that I know that I NEED the downtime/relaxation built into my life, so as to be able to function & remain balanced. Otherwise, like I said, it starts taking a toll on me physically & mentally.
It doesn't mean I'm really ABLE to mentally relax all of the time, if I have a lot going on, but at least having that time to myself where in theory I could relax, is important and I build it in. Also, I have key hobbies/interests that are almost guaranteed to get me out of my head, even if only for an hour. As a general rule I construct my life so as to avoid stress - I'm very... proactive about that, and self-monitoring. But again, it's because I know when I become overwhelmed, it takes a heavy toll on me. Those 'overwhelming' moments are unavoidable sometimes (i.e. a few months ago when I was looking for a new apartment to live in, making those calls, looking at the places, signing the lease, then the logistics of the whole move), but as a general Lifestyle I plan these Relaxation and Down Time moments/days into each week -- preventative measure, really.