Without knowing the topic or class from which this sentence should be applied, one would assume that personal experience might be the best way to discuss this topic. Think about the people we live with or come in contact with each day. Family, friends and coworkers are the main people that we interact with and those who see us at our best and at our worst. It is easy for good friends to argue when both happen not to feel well at the same time--whether that is due to stress or sickness, it would not matter. It seems that the more comfortable we become in our relationships and comfort zones, the more likely we are to become irritate more easily. Psychology books might be the best sources for interpretation for this sentence as well.