Our thoughts are a perfect reflection of our state of mind and how we feel about ourselves on the inside. Resentment towards another, such as an ex-, reveals a resentment of ourselves: we resent how we allowed ourselves to be deceived, hurt or heartbroken. Anger towards another, like a rival or business opponent, is derived from anger with ourselves: rooted in jealousy, fear, and a sense of insecurity with who we are, questioning our ability to aptly compete.
Our thoughts reflect perfectly like a mirror to reveal the inner workings of ourselves; the subconscious perceptions that we hold about our individual senses of Self and all of those around us. The mirror-like nature of our thinking is not only beneficial insofar as it reveals how we really feel on the inside; it also allows us to become more self-aware and introspective.
It is a gift, indeed, because understanding our thought-processes and how they reflect our individual understandings of who we are on in the inside provides the unique opportunity to become better people. Ultimately, we can diffuse inner conflict and confusion and become pacifists of the mind.