Thanks for the replies everyone. Adding a little sugar fixed everything. I'm not a super taster, in fact, most of the time, I scarf down my food without tasting it at all. Anyway, when I used to live with a friend, I would always use their olive oil and balsamic vinegar on salads. It never tasted bitter. Now, I realized that its because the cheaper balsamic vinegars are just regular vinegars with sugar added. When I bought more expensive balsamic vinegar (not the authentic stuff, just like 15 bucks for the bottle), there was no sugar in it. Anyway, this troubleshooting process was helpful.