A central concept of systems thinking is 'mutual causality': that A gives rise to B and, in turn, B gives rise to A. You cannot have one without the other. In the crudest of examples, we eat the environment and we are eaten by it. A and B are an indivisible whole and the dynamics of each cannot be understood in isolation.
There are two basic dynamic relationships between A and B in a system, although actual behaviour can be a complex combination of the two: (i) positive feedback, i.e. when a change in one results in a similar change in the other; and (ii) negative feedback, i.e. when a change in one results in an opposite change in the other.
You can see positive feedback in many common human interactions. For example, an argument can rapidly escalate when A reacts badly to B, who likewise reacts in the same way to A, and so on in a worsening spiral. Negative feedback is when A's violent reaction is dampened by B's conciliatory response. It's a little confusing in this context that positive feedback can be a negative thing and vice-versa, but when it comes to feedback in systems, 'positive' and 'negative' refer to patterns of dynamic change rather than value judgements.