Your anecdote intrigued me, and I was curious if anything like the following had ever occurred to you. It probably has, since it seems obvious to me but I have been mistaken about such things before ;)

The only place it's ever happened to me is at bars where a guy will approach me and say something nice and I will say "sorry I'm not interested"

For many simply being at a bar means they think you want to socialize.

So a man mustered up the courage to approach you and said something nice, only to be met with

sorry I'm not interested"

Seems rather abrupt, and which can mean a lot of things, ranging from, 'you're too ugly', 'too old', 'too poor', 'too short', and 'too fat' and so on. You may even have been genuinely sorry, and were only there to meet others, didn't want to talk, there just to drink or simply to dance, recently broke up etc but that isn't always obvious, but the preponderance of negative implications can shake anyones confidence, in addition to being summarily dismissed without the courtesy of an explanation.

Does it surprise you then that

he will say "you're not that pretty anyway" or something more rude

as reaction to an inferred slight, rationalizing their counter rejection as you being pretty but unkind?

I usually just brush it off because it's the bar and they are always filled with drunk douche bags.

that's an interesting attitude, and probably true of many establishments.

And hey, at least they tried to talk to me instead of just coming up... behind me and grinding

That's generous of you to consider.

Many men seem to view bars simply as pools of interested and available women, while many women view them as places to be seen and hopefully interest the best man present. Either way it seems chance rules.

I can't find the post, but someone suggested a number of weeks ago, a good approach for (I presume men) bars was as simply a place to meet people and make new friends and acquaintances.

If men and women develop themselves and their own interests, maybe they will find someone who shares them more naturally and not to have to resort to frequenting bars on the off chance they meet the right person.