Why is it easier to separate a cold egg?
It's a simple matter of viscosity, when eggs are cold the white is less runny. When eggs are warm the white is much more runny. 

As for whether it is easier to separate eggs when cold or warm it depends on your method. If you use the shells to separate the white, by cracking them in half and then transferring the yolk back and forth between the halves, then I'd say cold is a bit better because once the white starts to drip out it tends to go as a whole. With warm eggs more white will stay in the shell with the yolk. 

However, if you use your hands to separate the eggs, by letting the white drip between your fingers (or using an egg separator which does the same thing), warm eggs work much better because the runniness allows them to flow easier. This method works fine for cold eggs too, but warm is faster. 

Of the two methods I prefer using my hands rather than the shell for a few reasons:


Less broken yolks: It's easier to break a yolk when using shells
It's faster: especially if your eggs are room temperature you can separate eggs twice as fast this way
You can whip the whites right away: Most of the time the reason you are separating the egg whites is to whip them into peaks, and warm whites whip better than cold whites. If your eggs are warm already then you can whip them right away, if they are cold you have to wait for them to warm up


A handy trick if you are separating a lot of eggs is to separate each white into a separate bowl, then putting the white into a larger bowl with the rest of them. This is so if you get a bad egg, or a yolk breaks into the white you can discard the egg and keep the rest of them.