Why was it Grissom's fault? She left because she couldn't sort out her issues about her childhood. It really had absolutely nothing to do with Grissom. If she had told Grissom she was leaving and he let her, then yeah, maybe I could see it would be partly his fault. But, he had no clue she was leaving until she was already gone. It may have been sad for Greg that his friend left, but it was much worse for Grissom considering that (no matter how much I hate saying this lol) he loved Sara and had a romantic relationship with her.
Greg may feel like, well if Grissom had never returned those feelings Sara had for him then maybe Sara would have stayed. But, that's not why she left. In all honesty, if it hadn't been for her relationship with Grissom, she might have left even sooner. Grissom took her mind off of her "demons". But, after a while, the demons got too strong and nothing could stop them from jumping back out at her. That one case about the abusive husband got to her because it reminded her of her parents. And then that stupid genius girl made things worse. Sara never got the proper help for all of her issues and then they just bubbled over after a while. She had no issues with Grissom. She left cause she didn't want to bring him down with her.
I wish they'd at least had her mention the rest of the team in her letter. Especially Greg considering they were supposed to be best friends. She could have told Grissom to tell Greg that she's sorry for leaving without saying goodbye, but it was something she felt she had to do for herself.