Plain 跟 walks alongside someone through any action. 跟2 needs a speaking verb after the person; the person is who hears, not who joins in.
✗ 他說了我一個故事。 → ✓ 他跟我說了一個故事。
✗ 我跟他高興。 → ✓ 我跟他說我很高興。
✗ 他跟我一本書。 → ✓ 他給我一本書。
English folds the listener into the verb ("tell me", "ask you"). Mandarin marks the listener with 跟 before the speaking verb; the bare verb cannot carry it.