Doubting that a person is honestly and accurately relating their experiences is one thing

And even that has it's place since people do in fact exaggerated their experience.

Well, no.

If someone says "Joe did bad things X, Y, and Z to me because he plays violent video games," we can take it as true that X, Y, and Z actually happened without thereby agreeing that video games have anything to do with it. Being unconvinced of the conclusion (or even rejecting it) does not license us to disregard the evidence of that person's experience.

See also Qiaochu_Yuan's comment here:

General note: let's not do that thing where we don't like an argument someone is presenting and so we fail to update on the evidence they present in favor of it.

A comment is supposed to be upvoted for making a relevant contribution. A relevant contribution to the discussion of a statement is more likely to be criticism than it is to be a presentation of further corroboration. So, the most upvoted comments are likely to be critical ones. Mere voicing of agreement will necessarily be perceived as less relevant to the conversation and thus receive fewer upvotes; to change that, the norms about upvoting would have to be changed, so that being nice is considered more valuable than making a relevant contribution.

Why is this unfortunate?