#and it's not just bc he cares abt Luffy. He's also putting his faith in him as the person who can save them. He'll shelter him from harm

[koalas-koalas-everywhere]

One Piece #758 (vol. 76)

Hang on, I need a second... Usopp, who usually takes credit even for things he didn't do, who respects Luffy's good opinion so much that he credited his faith in his crew as the reason they kept fighting, that he agreed to work with a random, dangerous Warlord in a plan to take out an Emperor of the Sea just because Luffy complimented him, saying it doesn't matter if he never finds out that Usopp saved his life, as long as he's safe and can keep going...

Not that it's unprecedented for Usopp to do something like this - in fact, it's one of the first things we see him do. He also kept Kuro's invasion of his home village a secret. And maybe I'm reading too much into this, but I don't think it's completely irrelevant that Syrup Village's existence such as it was (a peaceful and carefree place that, if anything, could use someone like Usopp causing mischief to liven it up every once in a while) depended on the secrecy. Here he just thought, "I saved you, that's enough".

(Not to mention the comparison - this would be putting Luffy on the same level as his home village, the place he grew up in.)