Er, that sounds suspiciously like the eternal question "which came first, the chicken or the egg?". If you start off with a "happy" state of mind because of "external" reasons, you are probably more likely to succeed at work too, but of course your work also should contribute to maintaining your happiness. So it's either a virtuous cycle of success at work (without having to invest all of your time into it!) leading to a better overall life; or a balance of work being (negatively) stressful and you needing to "recharge" in your spare time (family life, friends, hobbies...); or a vicious cycle of your work burning you out and also leading to personal problems - of course, a positive attitude helps, but IMHO asking the simplistic question "which comes first" is as dumb as in the chicken-and-egg example...