תורה מסיני. This Orthodox belief, literally translated as "Torah from Sinai," states that the Torah is Divine Revelation given from G-d to the Jewish people at Mount Sinai. I am not here to dispute its divinity. I am here to point out that the Orthodox take the belief one step further and believe that not only all of the mitzvot were given there, but all the interpretations (see page 2, passage from Intro to Mishne Torah). From there, the Orthodox assert that "this is how it has always been," and as such, will not change traditions.
There are other beliefs that bolster their religious conservatism that creates sentiments of not wanting to change, whether that is to add new Rabbinic decrees, take away the old decrees, or re-examine how we approach a certain Jewish practice. The one concept is the Talmudic view of the deterioration of intelligence. This is of importance because the claim here is that the further away we are from Sinai, the less we know the Torah. There are only appromximately a dozen passages dealing with the notion and come with ambiguities. In his book, Menachem Kellner describes the Maimonidean view of how such declination does not exist. I have issue with the belief not simply because intelligence has increased over time or that the Information Age has given us more access to Jewish texts than ever before, but also because the beginning of Pirke Avot (1:1) illustrates the perfect transmission of the Torah.
The other belief, in conjunction with intelligence deterioration, is that Jewish practice does not change because the law does not permit any flexibility. This [theoretical] flexibility was something I had explored while examining the laws of Yom Tov Sheni. Not only does Jewish law allow for changes, but it also always has been a part of the history of Jewish practice, as I am about to illustrate with Tu B'Shevat.
The earliest reference to Tu B'Shevat was in the Mishnah (Rosh Hashana 1:1). When there was still a Temple, a tenth of one's income was tithed to go the functioning of the Temple and to the poor. This tithing system also included a one-tenth tax on fruit. Essentially, Tu B'Shevat started out as a midwinter fiscal new year for fruit trees. With the falling of the Second Temple, there was no tithing to the Temple, and thus no Tu B'Shevat.
If that were the end of Tu B'Shevat, this wouldn't be a very fun case study to use. However, the mystics in fifteenth-century Safed (צפת) changed all of that. An anonymous student of the Ari, better known as R. Issac Luria, created a fifty-page pamphlet with a Tu B'Shevat Seder, mimicking some of the aspects of the Passover Seder and celebrating various fruits. As the previously cited Chabad article states, one of the main premises of this seder is to realize that "the flow of G-d's beneficence is called the 'Tree of Life' - the roots, above in G-d's essence," and that "by eating fruit on this day we rectify and increase this flow." As intriguing as this practice can be for some, we have to recall that this practice was only done in mystic circles, which have historically never been mainstream.
Therefore, most Jews didn't celebrate Tu B'Shevat until the late nineteenth century. During this time period, Zionism was manifesting itself. One of the ways that the Zionists of this time expressed their love for the land of Israel was to plant trees, which was important since Israel had essentially been a barren wasteland for centuries. The practice to donate funds towards organizations to plant trees, most notable the Jewish National Fund, is still observed today by many Jews as the "Israeli Arbor Day."
However, a fourth type of Tu B'Shevat emerged in the latter half of the twentieth century. The beginning of the environmentalist movement in the 1970's caused Jews to examine how they could make their environmentalist views more Jewish. They did so by redefining Tu B'Shevat as "the Jewish Earth Day" and extend the definition of the Zionist version into a more global one. Jewish environmentalist organizations such as COEJL, a more liberal organization, and Canfei Nesharim, an Orthodox group, both provide great resources on making the best out of the Jewish Earth Day.
Conclusion: If Judaism were some stagnant, unchanging, inflexible framework, Tu B'Shevat would be nothing more than a vague memory from antiquity. Rather than be a relic from the past, Judaism prefers to give us the ability to change and innovate when the opportunity arises. The fact that three types of Tu B'Shevat are still practiced today are an attestment to all of this. And who knows, we might see another type of Tu B'Shevat emerge in the future. So for whichever version of Tu B'Shevat you celebrate, may you do so with כוונה.