I understand this. It's like a self-protective mechanism......I find myself doing the same. I allow myself room for hope, but I find myself operating more on the principle of expecting to be disappointed if I allow my expectations to be too hopeful!
I sort of expect to be hurt or to have trust broken (and realistically, this sort of thing does happen, people aren't perfect and even friends will sometimes hurt you without having meant to....) even by the people I'm closest to. It has kept me from really just being at ease in my relationships, because even with my husband or my best friends, I'm still on guard for where the hurt is going to come from. Or I ask little because I expect little.....