Why do magnetic properties vary with temperature

The individual atoms (each acting as a tiny magnet) in magnetic materials tend to line up in the same direction producing a large combined magnetic effect. Increasing the temperature eventually produces enough random thermal motion to destroy the alignment.

Look up 'Curie temperature'.

Of course, magnetic properties vary with temperature. The atoms has a magnetic moment. In magnetic materials these atomic magnets (atoms having magnetic moment) align so that there is net magnetic moment in some direction. these alignment can be in the form of domains ( I am not going into those details). In non magnetic materials the alignment of atomic magnets are so that the net magnetic moment cancels.

So, when a magnetic material is heated, more and more atomic magnets starts align to eachother canceling the effective magnetic moment. that is why when temperature is increased, the magnetic effect decreases. In short, Colder temperatures will permit the magnetic field strength to increase a bit and hotter temperatures will decrease it a bit (sometimes more, depends on temperature and material).

Because your dick is more attracted to the heat.