For better or for worse, I have a nature where I challenge everything I am taught. That is the nature of Talmudic discourse and I occasionally dabble in the study of Talmud. So here goes....
We were all [probably if you are reading this] raised that Yom Ha-atzmaut is a GREAT day. A real Jewish-Zionistic holiday. Anyone who thinks otherwise is anti-Israel.
Is that true??
Let me give an example: You want to buy a piece of meat in the store. On the packaging it says that this meat is certified TREIF by the Chazon Ish, the Brisker Rov [Rav Yitzchak Zev Soloveitchik], Rav Aharon Kotler, Rav Moshe Feinstein, Rav Ovadiah Yosef, Rav Bentzion Abba Shaul, The Lubavitcher Rebbe [and every other Chasidic Rebbe], Rav Shach [the two actually agree on this one], Rav Elyashiv, the Steipler, his son Rav Chaim Knievsky and countless other luminaries. However, Rav Tzvi Yehuda Kook, Rav Herschel Schachter, Rav Nachum Rabinovitch, Rav Avraham Shapiro, Rav Aharon Lichtenstein and others say that it is kosher and that one SHOULD eat it. Would you eat it?
If one or more of the Rabbonim Hamachshirim was your personal rebbi, then you probably would. But wouldn't you feel uncomfortable knowing how strongly all of the gedolim felt that you shouldn't eat it? Wouldn't it give you pause to consider why they rendered this meat treif?
Now substitute the words "piece of meat" for "celebrating Yom Ha-atzmaut". Whether Yom Ha-atzmaut is a day of celebration is for religious people a religious question. With all due respect to the list of "machshirim", both in number and Torah knowledge, those who were opposed are widely acknowledged as the greater experts in Torah [I think the Rabbonim "Hamachshirim" themselves would admit that]. If they said it is treif then the accusation that by not celebrating one is somehow being an ingrate [as the argument is often made] or religiously flawed is a difficult one to support.
Rabbi Robert Zion, Rabbi of the Young Israel of Wayne County, is a greater servant of G-d than the Chazon Ish?? He "gets it" but the Chazon Ish [and just about all of the gedolim since] don't?? Yet, a lot of fifth and eighth tier Rabbonim give shiurim preaching that anyone who is not a flag waver is a sinner.
"Think for yourself" people say. Don't just follow your "gedolim".
Well, one can think for himself and decide that his doctor is a bigger expert in medicine and follow his instructions. In the same way, one can think for himself and let the experts decide questions in their area of expertise. Gedolim are experts in deciding halacha and minhag.
But to give a short explanation of their position on one leg: The major reason they were so strongly opposed is because the State is a rabidly secular entity. If someone was a REALLY great guy and saved my life 3 times and sends me a check every month for 10k but ALSO spits on my father and degrades him numerous times a day - I wouldn't like him too much [despite my feelings of gratitude for his help]. We love our Father in Heaven no less than our biological fathers and the trampling of the secular establishment on the Torah on a daily basis [as a secular Jew does - whether he views it that way or not and whether he is guilty or not due to his upbringing] make it impossible to celebrate the day that this secular entity announced their sovereignty.
So do as you wish - say Hallel, shave, dance, party, wave the flag [if that is your psak] but please, don't accuse those who don't celebrate as you do of being sinners.
There are hundreds of thousands of people who live in Israel or who love Israel but don't feel bound by Yom Ha-atzmaut as a day of national celebration.