hop bitterness, flavor, and aroma are derived from different compounds. The bitterness comes from isomerized alpha acids. Isomerization of alpha acids requires heat, in particular boiling temperature. So, the longer hops are boiled, the more alpha acids are isomerized, the more bitter the beer.
Hop flavor and aroma come from compounds in the essential oils of hops (e.g. myrcene, humulene, etc). These compounds are quite volatile, which means that they are easily vaporized (even at temps below the boiling temperature of water). The longer you boil hops, the more of these compounds will vaporize and exit the wort, taking their flavor and aroma with them. Aroma compounds are the most volatile (that's why they're aroma compounds to begin with), so these are lost the quickest.
So,
longer boiling --> more bitterness due to more isomerized alpha acids and less flavor/aroma due to volatilization of the essential oils
shorter boiling --> less bitterness due to fewer isomerized alpha acids and more flavor/aroma due to reduced volatilization of the essential oils
this is why many recipes call for multiple hop additions at various times - thereby capturing bitterness, flavor, and/or aroma