Celebrities, being people, have a tendency to die. And when they die, two things usually happen:
1) There is an outpouring of grief, both from fans of the celebrity and from people who are just sad to hear of any death;
2) There is a backlash, wherein people - some well-meaning, but many just bullies - start ranting about how people shouldn't mourn celebrities because [pick one: people die all the time, you should be worried about x other societal problem, that celebrity wouldn't mourn if you died, etc.]. Often this is accompanied by some broad statement about how sad it is that celebrities are so valuable in our society.
I wasn't a huge fan of Whitney Houston, but I remember looking up to her as a kid because she was one of the few people on the radio who could really sing. I remember her as one of the few black role models I had as a white kid in a mostly-white community, in a town where black kids and white kids didn't play together. (Black kids need black role models so they know they can succeed; white kids need black role models so that respecting a black person does not seem foreign to them.)
When a celebrity dies, and people mourn, we're not just mourning someone who sang pretty songs or made entertaining movies. We're mourning a human being whose life affected ours in some way. We're paying respect to the fact that those pretty songs, those entertaining movies, or that inspiration to be more than we previously thought we could be - those came from a person. We're acknowledging that this person lived to provide us with entertainment, and in many cases died because of it. Most of all, we're recognizing that a human being has died, and not just any random human, but one we knew things about. One we watched for years, often seeing a struggle against drug addiction, or domestic violence, or mental illness played out on international television. If we have any conscience, we watched with sadness and with some hope that they'd pull through and have a happy ending. And if we have any conscience, when we learn that such a happy ending is never to be, we mourn.
And if there is any broad statement about society to be made here, I would say that it is this: It's sad that we live in a world in which some people are deemed to have deserved to die simply because they were human and made weak, vulnerable, human choices sometimes, just like we all do.