Look, I ain't in this for your revolution, and I'm not in it for you, Princess. I expect to be well paid. I'm in it for the money!
That was the crux of the issue between her and Han from day one, and it remained so three years later.
Leia knew he couldn't be 'in it for the money' anymore. The Alliance coiffures had run low enough that he'd stop taking payment in actual credits a long while back, and even before that, the piffling amount they were paying him was nothing compared to what he could make smuggling. And that wasn't even getting into the fact that he'd quietly given back the reward for rescuing her.
So, no, Han Solo wasn't exactly the mercenary he'd have everyone believe. He was deeper than that. He felt things. He cared. It was his dirty little secret – though a poorly hidden one. Just a little time spent in his company gave him away. It also didn't help that he left evidence everywhere in his wake; the mere existence of her, Luke, Chewbacca, and the entirety of Yavin IV the most publicized example.
All of that made Han a far better man than he believed himself to be, but none of it meant he was in it for the Rebellion. At best, he was here for his friends; at worst, because he had no better alternative for hiding out from the many bounty hunters on his tail. The most frequent sentiment out of his mouth – the one thing Han Solo had made sure everyone on every base they went to knew – was that he was his own man, independent; he wasn't a part of the Alliance.
Still, Leia often wondered, hoped, believed he must be to some degree in it for the cause if he chose all this time to scrape and struggle beside them, fight beside them, risk his very life beside them.
Yet, he continued to hold back. And therefore, difficult as it had become, so did she.
Of course one word, just the smallest sign, from Han of an intention to officially join up would put an immediate end to her struggle. She wanted him – and if he would only just sign on with the Alliance she could have him. What a relief that would be, finally surrendering to what her body had wanted for years.
Han too at the very least wanted her in his bed; Leia was sure of that. And he could have her. She wanted him to have her….
If only he would just enlist!
Commit to the one thing she was committed to most. And above all else, commit to stay.
That's what signing those enlistment papers truly meant: a promise not to leave her.
Without that, she simply couldn't give in to what they both wanted. She'd lost so much already. Everything. She couldn't put herself through losing him, too. On top of everything else, it would be too much to bear.