Sting is right on in his response. All honey does crystallize, the nectar source plays a major role and the temperature at which you store your honey speeds up or slows down the process.
One thing I might add is that when you process your honey keep it covered as much as possible. Not only does honey absorb moisture from the air, if left uncovered any dust particles that get into it will only speed up the crystallization by becoming the 'seed' that Sting spoke of.
Oh, I guess there were two things. I have read that since microwave ovens are notorious for creating uneven heating patterns, it is recommended that they not be used for trying to re-liquify honey. They say that the uneven heating can harm the flavor of the honey by heating some of the honey to a too high temperature. I personally have never tried to use a microwave for this, so I can't verify or refute this, I'm just passing on what I have read.
A beekeeping friend of mine stores his honey in an outside, below ground cellar (temperature in the low 50's) so that his honey will crystallize. He says it's so that he has crystallized honey when he needs it and he can always liquify some if he needs that.