Hmm... again, it's the whole apples and oranges concept. Each of the backstories had different ways of being bad. I'm not sure what's lacking about any of them, apart from general happiness. Chopper had the old lady, but he was also shunned by his fellow reindeer for his blue nose, then attacked by humans once he ate the Devil's Fruit. He had the sadness of being misunderstood, persecuted, and the only one of his kind since birth. Then, he experienced the extreme guilt from accelerating the death of his mentor, Hiruluk, by accidentally giving him a poisoned mushroom.
Robin's sadness is quite similar in many respects. She was also shunned, although she went through the eradication of an entire village and constant persecution afterwards. She tried leaving the crew for their benefit, which is exactly what Nami did in different circumstances. To some extent, the crew members have experienced a piece of each other's pain. Even though Brook doesn't have Robin's past, the fall of his entire crew to disease has comparable sadness to the destruction of Robin's village. At the end of the day, loss is loss. It's what makes Zoro keep his promise to Kuina and Sanji keep his word about feeding starved individuals at sea. Even Usopp's compulsive lying is deeply rooted in his past as an orphan, but he turned it into a great ability for stories that made Kaya happy.
The Strawhat crew is so spectacular because each of them can comprehend each other's pain based off of their own experiences, even if they haven't had the exact same tragedies. And Luffy's da man for making sure none of his nakama return to those dark places ever again. Oda's construction of their friendship is a true masterpiece.
