Well for one she's always harbored a semi-secret desire to tap that, but timing has never been right for it to actually happen and she's perfectly fine with it. She likes the relationship that they have now. She considers Sam to be one of her best friends. He's always been so sweet to her, and since supernatural shit started happening to her he's been kind of an anchor in the sea of fucked up. She trusts him enough- when separated from the consequences of putting him in danger- to spill her guts about everything. Before she hid some things so he wouldn't get in trouble. Now she can just let it flow.