Their Captain was always there for them.
Luffy always graced them with his large grin, talking about everything and anything as he stared at them with wide, child-like eyes. He would always laugh with Usopp and Chopper, playing games and fishing with the two. He would always sit next to Robin, eyes closed as he listened to her read a book of mystery and adventure. Sometimes, he would sit silently in the kitchen, content with just watching Sanji work his magic. Other times, he would lay down next to Zoro on the deck, simply enjoying the calmness of the ocean, the waves lapping gently on the hull.
Luffy would blabber on to Franky about things that he could make and watch in unveiled awe as it was made in front of his eyes. And, every time he couldn't sleep, he would go to the one person who couldn't either, asking Brooke to sing him a song about pirates and plunder. He loved going to Nami, asking about all the places and the people she met before she joined his crew.
And, whenever they cried helplessly inside, their captain saw through their mask and did his best to comfort them, saying, "It's alright 'cause I have the best in my crew! You're my nakama, right?"
And he would be there when they were sick, worrying over them feeling as if he could do nothing but watch as they battled with the internal enemy. However, when they woke up, he was always there by their side, bright grin lighting up the sky and dark eyes happy to see them up and about again.
Because he was their Captain and they were his family.