With geared bikes the rear driveside (right) spokes normally have the highest tension on the bike and the rear non-driveside spokes have the lowest tension. The front wheel spokes should be equal on both sides with tension a bit less that the driveside rear. The tension differential on the rear wheel is due to the asymetrical hub flanges to make room for the cassette or freewheel cogs. With conventional 8, 9, or 10-speed wheels the non-driveside spokes will have about 65% as much tension as the driveside spokes. This is necessary to center the rim.
Loose spokes break due to excessive flexure, usually in the "J" bend at the hub.
Al