Tu B'Shvat is a celebration of continuity. After all, what says "I am here for you just as I was here for your fathers, and I will be here for your children just as I am here for you" like a tree. In some cases, it takes longer than one lifetime for a tree to come to "fruition."
The Talmud tells us that by this point in the year, the majority of the rainfall to come during the year has already arrived. Therefore, the trees have already started to grow, and this is the time when fruits begin forming on the trees. Because the fruits begin to grow at this time, it is fitting that we start the New Year for the tree (which has significance to the fruits produced and the gifts the fruit are subject to) at this time.
In Eretz Yisroel, the 15th of Shvat is the day when new sap starts to rise in the trees. It is a time of rejuvenation. It teaches us the important lesson that even in times that seem darkest, there is new life, in times of sorrow there is hope, and in times of Galut, (exile) there is the light of Moshiach.
There are varied customs regarding eating fruit on Tu B'Shvat. Some have the custom to eat the seven species of fruits that grow in Eretz Yisroel. This "Top Seven" selection is based on a verse in Devarim: (8,8) "...a land of WHEAT and BARLEY and (GRAPE) VINES and FIG trees and POMEGRANATES, a land of OLIVE trees and (DATE) honey." Others have a custom of eating fifteen species of fruit (the "top 7" and eight more). In today’s "global fruitopia," where fruits from all over the world are available from our grocers, we mix it up: starting with the "top 7", we move on to local fruity favorites and throw in a "new fruit" (that we haven't eaten this year) in order to be able to make the Bracha (blessing) of Shehechiyanu.
What better way to celebrate the birthday of trees than to actually plant a tree. There are all kinds of organizations and groups that are dedicated to planting trees in the forests of Eretz Yisroel. You can also plant a tree in your own neck of the woods if you want to. Tu B'Shvat is the perfect time to protest deforestation and the shrinking of the rain forests, although there's no particular mitzvah to do so.