Why is Aunt Polly so happy after checking Tom’s jacket pocket?

Aunt Polly is happy because the bark in his pocket, as well as the message, proved that he hadn't lied.

“The bark I had wrote on to tell you we’d gone pirating. I wish, now, you’d waked up when I kissed you—I do, honest.”
The hard lines in his aunt’s face relaxed and a sudden tenderness dawned in her eyes.
“Did you kiss me, Tom?”
“Why, yes, I did.”
“Are you sure you did, Tom?”
“Why, yes, I did, auntie—certain sure.”
“What did you kiss me for, Tom?”
“Because I loved you so, and you laid there moaning and I was so sorry.”
The words sounded like truth. The old lady could not hide a tremor in her voice when she said:
“Kiss me again, Tom!—and be off with you to school, now, and don’t bother me any more.”
The moment he was gone, she ran to a closet and got out the ruin of a jacket which Tom had gone pirating in. Then she stopped, with it in her hand, and said to herself:
“No, I don’t dare. Poor boy, I reckon he’s lied about it—but it’s a blessed, blessed lie, there’s such a comfort come from it. I hope the Lord—I know the Lord will forgive him, because it was such good-heartedness in him to tell it. But I don’t want to find out it’s a lie. I won’t look.” She put the jacket away, and stood by musing a minute. Twice she put out her hand to take the garment again, and twice she refrained. Once more she ventured, and this time she fortified herself with the thought: “It’s a good lie—it’s a good lie—I won’t let it grieve me.” So she sought the jacket pocket. A moment later she was reading Tom’s piece of bark through flowing tears and saying: “I could forgive the boy, now, if he’d committed a million sins!”