Early arcs Zoro and Nami -sometimes- act like the responsible adults compared to the rest of the crew. They are something between their reluctant parents and their babysitters. And more than anyone else they act like that towards their captain, Luffy.
Taking care of the small kids ( Usopp's pirates) and the big kid ( their captain Luffy)
On Whisky Peak, they were the only two who weren't fooled by the citizens' fake hospitality:
Luffy had them both stressed about when he entered the giant snake.
The way they complain about him and being worried at the same time reminds me of parents who deal with their mischievous child:
And it reminds me this beautiful photo from the cast of Opla where both Mackenyu and Emily are holding Iñaki's hands.
I could see their manga counterparts do the same to prevent Luffy from doing something extremely silly.