How would you go about explaining the passing of a loved one to a child?

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When my brother passed away at the age of 45 of congestive heart failure He's young son asked me why he had to die and leave them? That question Broke my heart for this little boys pain. I told him That his dad was very sick and he fought hard to stay alive but he reached a point where he couldn't fight anymore so he had to let go. I told him that he loved them very ( He, his mom and Brother), much and didn't want to leave them but he just couldn't fight anymore. And to always remember that even though his dad is not with him in body he will always be with him in his heart, and spirit no matter what he does or where he goes. After this he seemed to have a better understanding of his dads passing. Did I explain this wrong to my nephew? Should I have giving him a different explination what would you have done in my place?