Why do some mothers try to make other mothers feel bad?

I'm so sick of it! Everywhere I turn, there is someone trying to give advice or telling me what I'm doing wrong with my daughter. I'm SO sick of it. If it's not his mom or aunts, its my mother, or my nanny. All of which have had lots of children but, why do they insist of criticizing a new mother? I think it's beyond "there just trying to be helpful". It feels like a blatant attempt to put another mother down. I dont claim to know everything about being a mother but, my god, whatever happened to "the sisterhood". These are all older women who do this to me also. I guess since they are "seasoned pros", they forget what it feels like to me a new mother. Why cant we all just stick together and support each other. You can still offer advice but, not so negitively. My sitter actually told me that I'm starving my daughter because I'm not giving her baby food yet. She turned 3 months last week!