Micro happiness is a state of pleasure derived from small, fleeting moments of personal joy and a sense of accomplishment. These moments can be achieved by such trivial things as winning a video game, successfully cooking a difficult meal, driving to work and hitting nothing but green stoplights, enjoying a really good cup of coffee, cleaning one's house to perfection, etc. These are all temporary accomplishments which only impact the moment, have little bearing on one's entire life as a whole. They trigger endorphins, a critical brain hormone which causes temporarily bliss and tranquility. Daily rounds of micro happiness are are emotional landmarks for helping a person get through the day with satisfaction, and deemed critical to one's sense of self worth.
The larger accomplishments of life would induce states of "macro happiness," such as marrying an ideal mate, having a healthy well-adjusted child, landing a dream job, overcoming a deadly disease like cancer, winning a major award in one's professional field, etc.
An unhealthy preoccupation with pursuing micro happiness can become a trap for low income people whose life is so limited that pursuing the larger goals (macro happiness) is close to impossible. So they might abandon any hope of striving after macro goals, and instead settle into a never ending, daily ritual of chasing after an assortment of micro goals. They become reliant on the endorphin-producing moments of micro happiness they temporarily trigger.
She successfully replaced the dryer belt all by herself, and the feeling of pride and micro happiness it induced lasted all day for her.