2.) Voluntary
Usopp didn't know if it was even voluntary or not - when he did things like pull her away from an incoming attack, or grab her hand to sprint away, or latch onto her in moments of panic. When he touched her.
And likewise, Nami didn't know if it was intentional or not when she held onto him even tighter.
The way they shifted towards one another when surrounded by a crowd, friendly or not, or the way they would find themselves shoulder to shoulder late at night, confessing secrets they hadn't realized they'd been hiding… these things all happened gradually over time.
For two people so well-versed in the art of bullshitting, neither seemed to be able to help but drop all pretenses around the other. They'd created some sort of habit of experiencing extreme emotions together.
Emotions like self-doubt. The fear of not being good enough. The shame of being weak. Before they ever knew each other the way they did now, she'd gone to him and pleaded for him to make her stronger with a new weapon. So she could fight alongside the rest of the crew. A lifetime later, he'd finally broken down in front of her, beaten and bloody, sobbing that he was so weak, that he was too ashamed to face the others, that he'd lost the money that was so precious to her - to them all.
For them, for some reason unspoken, there couldn't have been anyone else to hear those words.
The first time she'd been separated from him, she'd cried, yelled at Luffy to go find him, to bring him back. Back to her. Because without him, she was missing something crucial. It'd been like her left arm was suddenly taken away. The second time, the separation had been so long that she couldn't help but crush him in a veracious embrace the moment she saw him again. Had that been intentional?
She'd felt like she'd lost part of herself and was finally getting it back.
She'd felt complete again.
It felt like gravity was pulling them together.
Neither had decided for things to be this way. But now, after growing up feeling so alone, so different and separated from the people they'd lived around, so isolated in the homes they'd raised themselves in, they had found, for the first time, someone that knew that exact feeling. Not someone who sympathized, but empathized.
So maybe it wasn't voluntary. Maybe it wasn't intentional, how they loved each other the way they hated themselves. How they blamed themselves the way they immediately forgave one another. How they lied to everyone else, but when they were around each other, found they could only try to pull the wool over their own eyes.