...how far back are we talking here? I'd love to go back to a time when I was eight or so, back when I didn't have a care in the world. I wouldn't want to live my teenage years over again - too many problems, both real and imagined - but being a kid again could be quite fun, especially if I was mature enough mentally to appreciate just how damn lucky I was.
Although I'd probably get bored of it after a while; it'd be far too easy. I can't remember the last time I was actually happy, but those rare moments of contentment in the present probably mean more to me because of their scarcity. There is less of life in the present for me to appreciate, but what little there is I appreciate all the more for it. Nothing to do but push forward, I suppose.