Why is it that I always feel the need to compare myself, my career, my education, my LIFE with that of others?

Why do I always get depressed when I see things on face book about people bragging about buying their new house, getting their new car, having such a wonderful day at their fantastic job, buying the nicest clothes, always going on vacation, always going to parties and weddings and get-together's with friends and family over the weekend, always posting pictures of them having such a grand old time.......

Why can't I feel happy for them and not depressed about what they have that I don't? Why do I feel like I am not as good as some other people? What makes their lives so much better than mine that I can't just be happy in my own skin?

I feel as though I have worked hard to get where I am in life and I am continuing to work hard to get towards my goals in life, but I am not there yet and I have quite a ways to go. Some other people seem to already have accomplished so much AND THEN some. I actually go out and try to buy the same clothes as certain family/friends that I have that I think look nice and try to style my hair the same way, do my makeup the same way- It's almost as if I am trying to be like them because their life seems so much greater than my own and when I step back and admit it, it just gets me so depressed. I feel as though I am not living my life to the fullest because all I do is compare my life to others and view my own as dull.

Anyone have any suggestions? I deleted my face book account because I am sick of seeing how "perfect" their lives are.

Thanks in advance.