Difference between complaining and sharing

Hello everyone! I work in a school and yesterday, with a colleague of mine we noticed that some of our colleagues often come to our office to complain about their work life, family, personal etc ... Since they are in the same office as this colleague of mine, it often happens that we talk of personal things that afflict us but, comparing ourselves, we have always experienced this externalization as a comparison and a sharing of the other and never as a complaint. So I was wondering, why with some people are we inclined to perceive a conversation as a complaint and with others as a sharing? What are the characteristics that differentiate a complaint from a share?

Thanks.