It is a widespread prejudice that sweet wines are sugared or otherwise sweetened and that they are less agreeable than dry ones.
Ripe grapes contain two kinds of sugar: fructose and dextrose. During the fermentation process both of them change to alcohol, the dextrose fermenting faster than the fructose.
Raw wines therefore mostly taste completely dry, because the whole of their sugar was fermented to alcohol.
To be able to bottle sweet wines, we prevent a part of the freshly pressed grape juice from fermenting. For this purpose we filter it sterile and store it cooled and under pressure.
Not-dry wines are blended to taste with this grape juice before bottling.
Occasionally the fermentation of a wine stops unintended. These wines keep a part of their natural sweetness. It is also possible to stop a wine during its fermentation on purpose.
Usually sweet wines have a lower alcohol content than dry ones. This is because the natural sugars of dry wines completely changed to alcohol during fermentation whilst the sweet wine still keep a quantity of their natural sugar content, or they have been blended with grape juice to sweeten them.
The question if better to choose a dry or a sweet wine simply depends on the personal taste of the wine lover. If you love sweet wines, there is no reason at all to drink a dry one. If you love dry wines, you will not really enjoy a sweet one.