I think that sometimes it can be a simple question of ownership. Somebody else's problem is their problem and you don't have to take it on board, or take responsibility for it. Somebody else's problem isn't a reflection on you so why let it get to you and bring you down? It's not worth it.
I do realise by the way this is somewhat harder in theory than practice.
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-M&S
?Two men looked through prison bars; one saw mud and the other stars.? Frederick Langbridge