Most people observe a well-known minhag to eat Milchigs. Some have their cheese blintzes or cheese cake prior to a main meal (avoiding halachic hard cheese which would necessitate a 6 or 5 and a bit hour wait). Others have one Milchig meal on the first day. The Minhag in my father’s house is to have only Milchigs for the entire Shavuos; others from Poland also share this Minhag. If and when I mention this to others, they look incredulous. Sometimes, they will say, “But you have to eat meat on Yom Tov” while others will say “אין שמחה אלא בבשר ויין”.
I mentioned the Minhag to Rav Schachter, and he confessed that he too had never heard of it. He did note that according to the Chafetz Chaim, though, wine was now the main ingredient for שמחה and so he felt that יש על מי לסמוך and I was entitled to continue this practice.
The relevant sources are Pesachim קיט and ,ביאור הלכה, או”ח תקכ”ט ב
תניא רבי יהודה בן בתירא אומר: בזמן שבית המקדש קיים אין שמחה אלא בבשר שנאמר “וזבחת שלמים ואכלת שם ושמחת לפני ה’ אלהיך”, ועכשיו שאין בית המקדש קיים, אין שמחה אלא ביין, שנאמר ויין ישמח לבב אנוש
והאנשים, בזמן שבהמ”ק היה קיים כשהיו אוכלין בשר השלמים… ועכשיו שאין בהמ”ק קיים אין יוצאין ידי
חובת שמחה אלא ביין… אבל בשר אין חובה לאכול עכשיו כיוון שאין לנו בשר שלמים, ומ”מ מצוה יש גם באכילת בשר כיון שנאמר בו שמחה [כן מתבאר מדברי הב”ח וש”א], והמחבר שלא הזכיר בשר אזיל לשיטתיה בב”י ע”ש, ולענין יין סמך על מה שהזכיר בס”א שצריך לקבוע סעודה על היין
In summary, the meat (not chicken) that is originally referred to is the meat of Korbanos. In the absence of Korbanos, men were required to institute their שמחה through the consumption of wine at the meal. [Women on the other hand obtain this through a Yom Tov gift].
My reading of the above leads me to a number of conclusions.
- If you make Kiddush on Yom Tov with Grape Juice and consume no wine, it would seem you have not fulfilled Chazal’s happiness requirement
- Kiddush wine does not constitute the type of drink, in my opinion, that Chazal were referring to. The sweet thick molasses that parades as Kiddush wine may serve the purpose of Kiddush because it has a name/שם of wine. However, I don’t see how anyone could consider it as an ingredient for שמחה. I’d go further, if it wasn’t called “יין” it could be cogently argued that it was not חמר מדינה (a regular drink of choice in one’s locale) because nobody but nobody would casually serve this to a guest who occasioned one’s house.
If it wasn’t for קידוש would anyone drink the stuff?
pitputim 
I don’t know if any of the wine we drink would be appreciated by Chazal. Their preferred drink was served highly diluted – do you dilute your wine to one in three or four parts? I have a theory that Chazal drank wine that was deliberately flavored with resin, perhaps myrrh, which is why Chazal say that, e.g., “חייב איניש לבסומי בפוריא”. Anyway, the Shulchan Arukh explicitly says that one can (in theory) squeeze a bunch of grapes and make kiddush on the juice immediately.
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It’s not relevant what Chazal drank in respect of my point about Simcha
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Eliezer Brodt recently posted an article on the Seforim blog. In it, he makes reference to Rabbi Moshe Dinin who evidently collected 160 reasons (!) for the Shavuot custom of dairy in his Kuntres Matamei Moshe (2008).
It might take you a while, but you might be able to find an answer to the genesis of your family Minhag in that sefer . . . .
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