Beethoven's music has three distinct phases - early, mid and late. This is very uncommon, since most musicians have a "set" style through out their career. Fifth symphony was written during the "mid" part of his career.
Beethoven was working on the fifth for four years.
Beethoven had started going deaf around this time, that may be the reason parts of the symphony are "loud"
Four letter motif of the Beethoven's Fifth symphony was used by BBC during the second world war because :
- It sounds like morse code letter "V"
- Roman numeral for five is "V"
- "V" for victory
Reference : Classical classroom podcast Episode 72