Law.
Temperance Brennan is in great demand as an expert witness. Everyone wants her in the seat. Her book means that the jury has likely heard her name at least on TV if they haven't already seen her face on the back cover of her latest novel. To that end, she's an impressive asset to whichever side has enlisted her help.
Temperance Brennan, however, has grown to dread the subpoenas. Whereas she once thrived amidst the courtroom mahogany, drunk on truth and the excitement of civic duty, her role as witness has changed. Brennan knows enough to confirm, deny, nuance, and complicate the evidence in front of her. She is not intimidated by an audience as she does what she knows she does best. In all honesty, she sometimes indulges her desire to put on a show. Exhibit A replays in her memory as she crawls into bed and allows her pajama-clad self to smile at the practical application of years of dedication.
The problem with Temperance Brennan is that now she has seen too much. An expert witness doesn't see the crime committed, she looks at evidence after the fact. But Brennan has seen many crimes now. They happen in front of her face. Her friends have dropped to the ground in the safety of her own lab and she has witnessed it. The years grow longer, and she becomes an unwilling expert in witnessing.