It's not what coffee is, but how it's brewed that makes the difference. A darker roast will often be sweeter than a light one. The darker the coffee roasted, the more intense its flavor will be. The natural oils in beans are released when they’re heated up in specific temperatures which is why dark roasts have a sweeter taste whereas light-roast coffees don't necessarily offer that same level of sweetness because it's still too early in the process. What you'll find is that bitterness and acidity levels also increase with lengthier roasting time, so to compensate for this lack of sweetness in lighter-roasted coffee drinkers prefer adding extra sugar or dairy creamers to their cups.
The type of chicory in the coffee can also affect sweetness—some people like to add sugar or honey themselves. Finally, freshness plays into taste too; stale coffee will always taste worse than fresh brews because it develops unpleasant compounds called ketones, which create acidity and bitterness in the grounds.