My mum specifically.
She is so negative. About so many things. My dad is quite like me - generally upbeat, positive person who assumes the best. But mum can make a minor problem into a major disaster - and always someone's fault (usually my dad's 
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Over the years most people have developed a wary relationship with her. She was outrageously rude to my SIL when she and DB first got married - SIL tolerates her now and their relationship improves as it's purely superficial. She hated my dad's entire family (with some reason I must admit). She took a long time to accept DH but once our relationship was a fait accompli she just made the best of it. She meets new people, thinks they are going to be best mates and then gets all intense and when they back off a little she suddenly discovers that actually she doesn't like them at all 
She constantly recalls bad things that happened to her decades ago - even things that happened when she a child, but rarely remembers the positive things.
If one of my kids misbehaves it's a sign that he/she is 'naughty' or 'selfish' and she won't hesitate to tell them so. And if they behave well there is often some ulterior motive to their behaviour.
People's motives are often base in my mum's world - she always sees the worst.
Now she has had the shitty end of the stick at times throughout her life. And I love her dearly and do feel quite protective of her, but it is so wearing. To always have to tiptoe round her, play the clown to keep her happy, try to make things as easy as possible for her.
She isn't always like this - she can be good company. I feel so sad for her too. Life is pretty amazing - but she doesn't see it like that.
How do you cope with this?