I'm naive but I really think personal happiness comes from pursuing the things you love, both personally and professionally. If you want to be a high energy PhD in physics, go for it! I love being a physician, so the long hours and all of that didn't bother me.
I do think that if you are ambitious and have some talent, as most gifted kids probably do, the sky is the limit!
There is a perennial overproduction of PhDs compared to the number of jobs available for researchers and academics. In your early 20s you may be guided primarily by intellectual curiosity, but when you are older and need to support a family, a string of 2-year post-doc positions or being paid $1000 to teach a course as an adjunct stinks. I suggest that only the very best college graduates -- people who were stars in their department as undergrads -- should try to get a PhD.
