i find that passionate certainty can be a good thing in some cases, especially when someone really does know what they are talking about.

but fanaticism is more often a problem than not. fanatics tend to not really understand what they're talking about, or twist it to fit what they want it to be about.

> Fanaticism: Excessive enthusiasm, unreasoning zeal, or wild and extravagant notions, on any subject, especially religion, politics or ideology; religious frenzy.

note -- not talking about any particular "thing" here. just commenting about passion vs. fanaticism in general.