If someone is crying over a hangnail, it is easy to say their problem is meaningless when compared to the guy whose arm was blown off. At the same time, it is not right to get into the habit of dismissing and invalidating other people's experiences or demanding that they can't have a feeling or a reaction to something unless it meets certain standards.
There are people who think it is silly to grieve the death of a pet, because it is not a human being. There are people whose first reaction to anything someone else worries or complains about is to sneer at "first world problems". Some people seem to think that if they don't know anyone with a particular problem, that problem doesn't really exist. And there are those who minimize everything that happens to others because "it could have been worse". Imagining a worse scenario doesn't magically make this one acceptable.
If there were a truly objective scale for measuring tragedy, it might show that the worst thing that ever happened to my neighbor isn't nearly as bad as the thing that happened to me. But it is still the worst thing that ever happened to him, and he has the right to see it that way. His experience is just as real as mine.