why do we go through periods of time when we only focus on the negative??

i am a VERY blessed woman. my family has it's health. i have a son and husband who love me and who i love. i keep hearing wonderful things about the wedding i had a few months back. i have a wonderful extended family. we're going to buy our first home. i have great friends.

so, why am i only focusing on the negative??
i'm always complaining; i only see the clouds and not the silver-lining. what is wrong with me??!

am i alone in this?

sometimes we just got through periods when it feels like everything *****, even though it really doesn't.