When your mind changes, your brain changes, too. In the saying from the work of the psychologist Donald Hebb: when neurons fire together, they wire together—mental activity actually creates new neural structures (Hebb 1949; LeDoux 2003). As a result, even fleeting thoughts and feelings can leave lasting marks on your brain, much like a spring shower can leave little trails on a hillside.
For example, taxi drivers in London—whose job requires remembering lots of twisty streets—develop a larger hippocampus (a key brain region for making visual-spatial memories), since that part of the brain gets an extra workout (Maguire et al. 2000). As you become a happier person, the left frontal region of your brain becomes more active (Davidson 2004). {pg.5}
Because of the ways your brain changes its structure, your experience matters beyond its momentary subjective impact. It makes enduring changes in the physical tissues of your brain which affect your well-being, functioning and relationships. Based on science, this is a fundamental reason for being kind to yourself, cultivating wholesome experiences and taking them in.