If you are happy with yourself, you have a better chance of making the best of whatever circumstances you find yourself if. But there are plenty of external circumstances that would make any sane person unhappy, regardless of how they felt about themselves.
Absolutely. That is one of the reasons why I titled this thread "The theory says.........".
In practice, am coming round to the thought that
some people (and am not excluding myself from this!

) look outside of themselves for "happiness" how ever they may be defining it, when what they should be doing is looking inside.
So for instance... a new haircut, a new job, a new city, a new country (!) might bring that "happiness", when the
real problem may not be the hair, job, city or country.
Re external circumstances, some circumstances can be changed, it is a question of how much someone wants to be happy, that they will effect the changes necessary to be happy.
Where am I going/coming from in all this? No, not "unhappy" myself, but as previously said, the winds of change are ablowin' and I have been trying to reflect/contemplate which of those winds (not of the bodily type!) are from how I am feeling inside, which are external circumstances that are within my power to change, and which, if any, are not.
Where someone has chemical imbalances, health problems, depression and such like, I am classifying those as
external circumstances, even though those circumstances are internal to them because there is often very little one can do about the above without external help. So, for the purpose of this discussion

I would like to exclude those sorts of external circumstances please.