Because you have recognised this pessimistic attitude in your family..deal with it..Read books, look into finding a way to learn how you can be an optmistic person.There are tapes available. and they do work. Go on line and google for infromation on self help programs concerning optimism.
Disown them. They are toxic.
Tell them who you are, stop avoiding it, you need to tell them how they make you feel, it's not your fault if you hurt their feelings, they shouldn't be so negative so much duh