Simchah is related to wanting and having, because Ben Zoma defines the wealthy person as “sameiach bechelqo — happy with his lot”.
The Tanya speaks about how each aspect of the soul lives in tension between “ratzon - desire/will” and ta’anug. Thus we see that “oneg” too is related to wanting and having.
However, the mitzvah on Yom Tov is deemed simchas Yom Tov, whereas for Shabbos we speak of oneg Shabbos.
Simchah has codified requirements: for men, meat (some rishonim say that deOraisa it’s only the meat of the shelamim sacrice, but all agree that including derabbanan, it also calls for meat in general) and wine, for women, new clothing and jewelry, for children, sweets. The two differ.
Perhaps we can explain this in light of my previous entry which suggested that
… I think ben Zoma’s notion of my lot in life is the path Hashem placed before me to travel. Not where I stand now physically, socially, psychologically or spiritually. Not even where G-d is leading me. My lot is the trip along the way. The whole roller coaster ride, the peaks and the dips. … The job for which G-d created me as I am, when I live and where I live, with the people I know, the responsibilities I face, and the challenges He throws at me, solely because this is something His great plan required that required his having a Micha Berger to do it.
But in light of an Avodah discussion, I noticed that my notion also implies a possible distinction between simchah and oneg. The Tanya defines oneg as the satisfaction of a desire, the achievement of something one willed to accomplish. If simchah is satisfaction with one’s general life as a process, oneg is enjoyment of where I stand at the current point.
Rabbi Nachman Cohen, my principal as a Junior in High School, once defined Shabbos for us as “Shabbos is the island in time which is the eternal present.” Taking a break in the process to assess where one is going. Thus the greater cessation from melakhah, creative activities on Shabbos than on Yom Tov. (And even greater on Yom Kippur, where stopping to assess is even more critical.) It makes no sense to hurry up the ladder to get to the top of the wall only to afterwards realize the ladder was leaning against the wrong wall! Someone who looks back on their life with regret that they traded their role as parent to be a “success” at their career simply never kept Shabbos. And they never found oneg. Enjoyment of the accomplishments of the moment. Pausing.
All of this would imply that simchah requires more indoctrination than oneg. It is easier to take joy in what’s before you than in the more abstract concept of the path your life takes — including both triumphs and challenges. This would justify why halakhah defines exercises with which to express / internalize simchas Yom Tov in a way that it does not for Shabbos.
Perhaps this too can be explained in light of a point R’ JB Soloveitchik draws from Qabbalah. In Qabbalah there are two concepts: is’arusa delesata — the awakening [of holiness] from below, and is’arusa dele’eilah — the awakening from above. Shabbos happens every 7th day, G-d set it in motion, He is reaching down to us. It is is’arusa del’eila. Yamim Tovim depend on beis din setting the months. Thus, they are is’arusa delesata, from us up to Hashem. This is why the berakhah in the Amidah for Yom Tov is meqadeish Yisrael vehazmanim – who Sanctifies Israel and the [special] times”. The times’ holiness comes from Israel’s. For Shabbos, we simply say “meqadeish haShabbos“, no dependency on Israel.
Rabbi Soloveitchik explains this idea using the metaphor of visiting. On Shabbos, we come to visit the A-lmighty. Is’arusa dele’eila — He invites us. On Yom Tov, we invite Hashem to join us. Shabbos involves oneg because when you’re the guest, the Host provides things as per your desires. When you are the host, things are patterned around the Guest’s instructions — the more structured simchah.
I think this ties in. On Shabbos, Hashem invites us to take time to be “in the moment” to check the ladder rather than climb it. Thus, the mitzvah is oneg, happiness with the moment, and the more tangible kind of enjoyment. We are His guests, enjoying what He provides us. Thus, “sheishes yamim ta’avod — strive for six days”, and then take the time for oneg — to acknowledge what needs were satisfied. On Yom Tov, the focus is on His “happiness” (so to speak), and thus is about our role in His greater plan. It’s simchah.