I'm not talking about any kind of damage - unless you listen to your music way too loud, of course, though over the log term they can affect your judgment if they are too fatiguing-sounding.
By "fatigue" we're in this instance referring to how comfortable they are for you to listen to and for how long you can work with them at a sitting before you just want/need to give your ears a break. Some folks find some speakers to be maybe too harsh sounding or too boomy or too something for their ears and their tastes and plain get tired of working with them.
It's only basic sense that the more "comfortable" your ears are with the way the monitors sound, the easier it makes the task. But this goes hand in hand with your ability to actually make a mix. They can be as comfortable as a warm blanket on a winter day, but if you just can't get a good translation out of them, they're useless. OTOH, if you have a monitor that you can translate easily and make a great mix, that doesn't help any if you can't stand listening to them for more than 5 minutes at a time.
G.