In every moment of life, there's an underlying physical reaction to everything we experience, a subtle opinion that's expressed in the body: What has just occurred is "agreeable" "disagreeable" or "neither." For example, the subway door slides shut in front of us; before we manage a thought about the event, a physical editorial has already arisen in our autonomic nervous system and muscle groups: our breathing becomes shallow, the abdomen clenches, the shoulders constrict and lift, the jaws lock; missing the train is now "disagreeable." (This somatic reaction sequence, referred to as "vedana" by The Buddha, is virtually automatic, and has been confirmed by neurologists as a function of the brain's subregion responsible for survival, the amygdala.)
These physical commentaries are sculpted by a lifetime of previous reactions to various experiences: we've come to associate missing the subway with other uncomfortable events (such as being late for work); and so we unconsciously label the stimuli (missing subways) as threatening when it reoccurs.
Somatic reactions are often completely unnoticed by the conscious mind, for our awareness is hijacked by our first thought based response—“Shit. I hate this.”—which then develops into an obsessive, self-centered stream of delusional ideas: "Why does this always happen to me? [It doesn't, it happens to everyone.] Now my day will be ruined. [That's an nsubstantiated guess about what's unknowable.] The subway sucks. [Entirely subjective.] etc.” This is what is known in the dharma as "the chain of suffering (paticca samuppada):" After contact with an initial disappointing, all-too-inevitable event, we add more and more suffering to the experience (first physical, then a general thought reaction, then mental dialogue about being victimized by life). Stress and resistance spreads from the body to the mind; eventually we're thoroughly uncomfortable.
Fortunately there's weak link in the chain: between the arising of discomfort in the body and desire to be rid of it, we're capable of interrupting the sequence by bringing sustained awareness and compassion to the various "vedana" sensations: We note the discomfort as it arises in the body, swells, then subsides. We limit our thoughts to the simplest observation: "This is disappointment." We gently extend our breath exhalations to relax around the discomfort as it unfolds. In this way our attitudes and actions are not unconsciously produced by previous associations, but are examined by an awareness that can attend and respond with compassion; we haves derailed our habitual self-centered reactions.
This is the way out of day to day suffering: insight to the impermanent nature of phenomena gives rise to equanimity, a state where we can experience strong feelings without turning them into anything personal.