The more certain you are of how someone feels, the more you are attracted to them. Not only how they feel, but also how they are motivated. They conclude this from looking at patterns of neural activity.
But most importantly, the "attraction" is an intrinsic reward signal generated by your own brain from judging that it understands another person well. The other person can't actually alter that intrinsic reward except by making their actions easier to understand.
Well, it sorta depends on how the anxiety or low self esteem manifests itself. If the person is aware of their anxiety and doesn't try to cover it with over-compensation in the other direction (excessive self assurance or arrogance), then it can be rather charming.
Would it suggest that like-attracts-like with mental illnesses? E.g., if you have ADHD, other people with ADHD will be more predictable to you than people without ADHD, so you'll prefer to date them?