Apropos of some of the comments, there is a growing literature on positive psychology, which instead of training focus upon what's amiss, aims to move beyond "psychic entropy" (a cognate concept of existential angst?)
As to happiness, the ancient tao philosopher Chuang-tzu wrote to the effect that a superior means of finding happiness is to stop looking for it. What he actually said is, of course, open to analytic interpetation or outright rejection on any number of grounds, but its spirit seems to jibe with the quote above attributed to Albert Einstein. Getting enthusiastic about something (hopefully something moral and constructive) would be the opposite of lapsing into psychic entropy.
For what it's worth, in the German, Schopenauer wrote: "Ein Mensch kann zwar tun, was er will, aber nicht wollen, was er will." My non-expert literal translation is: A person can indeed do what he wants, but not want as he wants. In German, wollen is the infinitive of the verb to want, while tun is the infinitive of to do (an alternative of machen, or to make). Query: How do you interpret Schopenhauer's comment in the context of his work, The World as Will and Representation? (Die Welt als Wille und Vorstellung"?)
For us anglophones, to ...
There is a story of Carneades of Cyrene, a post-classical philospher, who came to Rome as an ambassador of Athens. As a member of the New Academy, Carneades was well versed in sceptical argumentation, and stood steadfastly against dogma of any kind. Once in Rome he proceeded to deliver a spellbiding address, arguing that justice should top a list of human motives. The following he day, in service of his real argument concerning the uncertainly of human knowledge (deep scepticism), he proceeded to contradict the argument given the previous day, arguing i...
"We must welcome the future, remembering that soon it will be the past; and we must respect the past, remembering that it was once all that was humanly possible” ~ George Santayana