It’s above all a natural phenomena, a sign of quality which indicates a honey that has not been spoilt by heating or pasteurisation.
Our honey produced locally in Devon & Cornwall is liquid when put in the jar & will cristalise a few days later (Spring flower honey) or in a few weeks if produced from summer flowers, such as Blackberry or Heather.

An industrially transformed honey is often liquid at purchase & remains so for a longtemps, sign of a poor quality honey

 

The speed of cristallisation depends on the glucose, fructose & water content.  

The temperature at which the honey is stored is also a factor
which modifies the speed of cristallisation.  At 15°C it cristalises quickly, at 20°C it remains liquid for much longer.

It is rare that a honey will remain liquid for months as in the case of Acacia honey.
 

However if you prefer to use your honey in it’s liquid state & it has started to cristalise, just place the jar in a a saucepan of warm water (38°C). this will maintain the excellence of the honey.