A solid wheel build should be pretty durable, but even then spokes can break if they get nicked or scraped, which happens sometimes when riding (especially on gravel or trail, where rocks can get kicked up and bounce off spokes). Its also easy to damage spokes if the drivetrain is out of wack and dumps the chain into the spokes over the cassette.
Once one breaks, it places additional stress on adjacent spokes which can cause a cascade of failure. Always best to replace a broken spoke as soon as you can.