What makes wine so attractive, interesting, and enjoyable is the amazing, complex array of flavors and aromas which individual wines produce. Each bottle of wine can be its own unique experience. If wines are all produced from simple grapes, though, how do they manage to produce so many different flavors and aromas? If wines are basically just fermented grape juice, why don't wines taste like grape juice? The answer to this question is the answer to what makes wine special.
Chemical Compounds in Wine Grapes
The first answer to this is that grapes are not "simple." No fruit is truly simple, but grapes can be especially complex. There are several different varieties of grape which are used to produce wine and all have their own characteristics in terms of sweetness, juiciness, and other flavors. Each grape tastes and smells a little different even before it is taken through the wine creation process.
On top of all this, the means by which wine is grown affects the flavors and aromas as well. Most farming is done in a way designed to help the produce grow and develop as much as possible but the production of grapes for wines is aided by stressing the grapes and preventing them from going as far as they might under better circumstances.
Extra stress concentrates the chemical compounds which give us the flavors and aromas we're seeking; less stress would produce larger, juicier grapes which are less concentrated and therefore less well suited for creating the best possible wines. You don't stress corn, apples, or pumpkins when growing them, nor do you stress grapes being grown for regular grape juice, but you do stress grapes grown for wine.
Terroir, Wine, and Grapes
If you spend any time around people who drink and talk about wine, you're bound to hear about "terroir" sooner or later — and probably sooner. Terroir is a French concept that can't be translated into any single English word. It refers to an entire network of environmental influences on grapes as they are growing: sun, air circulation, humidity, fog, temperature, soil, rockiness, and so forth. None of this is inconsequential when it comes to the impact on the grapes.
That said, terroir doesn't affect every variety of grape in exactly the same way. Some grapes are so strongly affected that wines created in the same manner but grown in different areas with different terroir can smell and taste remarkably different. With other wines, the impact of terroir is not quite so pronounced that it changes flavor and aroma so much, but it still has an impact.
Wine Fermentation & Aging
The final ingredient in what causes wine to have so many flavors and aromas is the process used by wine makers to create their wines. Just like a chef has a lot of different tools and methods to influence the flavors and aromas of the dishes they prepare, wine makers can do many different things to influence the wines they create. The more skilled and experienced the wine maker, the better the wines they can create with the grapes they are provided.
Among the things which will affect the flavors and aromas of wine are the length of time which the grape skins are left in contact with the grape juice, the length of time allowed for fermentation, whether the wine is fermented in oak or steel, whether the oak is American or French, whether the oak barrels are small or large, the environmental conditions in which the wines are fermented, and so forth. What a wine maker chooses to do will depend on the grapes they are using, what sort of wine they are creating, the purpose of the wine, etc.