Hi. I am very insecure. It has been one of the main causes of my major depression and anxiety for all my life. I think for some ISTJ's, they have such very high standards for everything and they are perfectionists that nobody and nothing can meet those standards, least of all themselves. ISTJ's are the ultimate practical people. They don't embellish themselves or disillusion themselves with false confidence or make up skills or strengths. They know what they are good at, but because even what they do, know and are does not meet their standards, this can lead them to become insecure and have a very low self-esteem. I also think it depends on life experiences. ISTJ's, from my experience of myself and other ISTJ's, value sincerity above most qualities. They will refuse to take compliments not just because they are modest and humble but because they truly do not believe them. So the only options are either another person brings them down and hurts them, or they give them a compliment, but because the ISTJ does not believe it for him/herself, that persons opinion is devalued. I hope this helps.