Ahsoka Tano was an island. She had no reason to let anyone in. She had her friends, the people who considered themselves close to her, but the girl would never let anyone truly close to her. Even Master Plo wasn't on this island of hers. He was just off the coast of it, on the brink of being close to her. No doubt she trusted and loved him like no one else, but it just wasn't enough. She wanted to let him in but just wouldn't - is physically pained her that she was unable to. If she was to be honest with herself, she was scared. Scared someone might breech her barriers, walk onto her shore, and betray her - stab her in the back. She couldn't trust her own family to make the right decisions for her; why should she trust anyone else? So she was an island. She was an island of hopes and dreams and solitude, one that had never been discovered by another. Until he came along.
Apprenticeship is a pivotal point in a Jedi's life. It determines what the rest of your career will be like, how you'll end up - possibly dead. When she saw the man who was to be her Master, she had no reason to trust him, no reason to let him get close to her island. Anakin Skywalker was just like everyone else. They think they know you when they really don't. Except he wasn't.
This man had a heart like no one she'd ever known. He didn't come off as if that were the case, but it was reality, and not everything is as it seems on the outside. Ahsoka saw a man, barely twenty, who loved fiercely and felt acutely, a type of person she never thought existed. Yet he did - he was as real and as hurt as she was, probably more so.
With that, her walls crumbled; her island became visible to him. Ahsoka, for the first time since the event that pumped mistrust into her veins, let someone in. She let her Master in knowing that doing so may very well be the biggest mistake of her life.
Yet she never came to regret it.