I think much of it is "I can't trust myself for fear of hurt".
We trust people all the time, every day of our lives. I trust that the driver in the car won't run over me when I cross his path on the cross walk, I trust that my colleagues will not sabotage my work, I trust that my doctor won't intentionally prescribe me something harmful, my hairdresser won't nip off the tip of my ear etc. etc. etc.
People are too limited in how they define "trust"... it's so often restricted to emotional protection. I think it's often a case of "I am afraid to make myself vulnerable (again)"