This can be caused by bacterial or viral infections, mucus coating due to post nasal drip, irritation of
box by irritants. Irritants can be anything from dust, smoke, etc.
Damage can also be caused if you suffer from acid reflux.
You may have damage to the vocal chords by over use, or by straining the vocal chords. If you do sing, then do make sure you do vocal warm ups.
The voice box needs to be looked after like any other part of your body.