From my experience, a large part of the reactions are unconscious, for example:
Anger: Can be based on traumatic experiences in the past, situation one was deprived, feeling helpless, or treated unfairly. Cases in which another person or group reacts in a similar way one experienced, e.g., as a child, can trigger the feeling of anger and fuel conflict.
Happiness: Many people are happy because the sun shines, because they are healthy because they have a job, a family, a working partnership, etc. Nevertheless, people do not ask themselves consciously: ‘Why am I happy now?’ What makes one person happy does not cause similar emotions or reactions to someone else. Understanding what makes one happy consciously, becoming aware of needs, can be crucial in mediation processes.
Frustration: Similar to anger, frustration is often linked to past experiences of deprivation, feeling alone. Also, when one has a strong wish or desire (again, usually not conscious where the desire comes from), rejection or failure can lead to frustration. However, one can also see rejection as a chance to do something else, transform frustration into happiness. 
Sadness: Similar to frustration. In many cases, it seems to be not clear for oneself or the surrounding why a situation or fact leads to the feeling of sadness.
Jealousy: People in Nepal, even though, especially in rural settings, often living under poverty. However, one can say that they still seem to be very happy with what they have, living a simple life in happiness. However, when due to natural disasters or climate change harvests do not bring enough to live from, tensions can arise, and motives like jealousy play a role in conflict and anger towards an inactive government increase. However, also when it comes to resources such as gold or oil, apart from jealousy, greed can play a critical role. One needs to reflect: why am I jealous? 
Sharing: Many people learn in their early childhood about the importance of sharing what they have. Communities lived together for centuries, sharing and helping each other is a foundation of society, especially in times of crisis. However, one who grew up in poverty, always fighting to survive, may not have learned this skill in childhood (or even more!). The motivation for sharing can be seen as something ‘human’ or ‘natural’ for some, but unlogical someone else. Understanding why wants to share or keep things for themselves can be trained through reflection.
Temper: Some call it a cultural factor, some a personal element. However, it is a fact that some people tend to have a high temper, lose their calmness faster, and often than others. Some might even perceive temper as aggression. From my perspective, temper is linked to emotions that are triggered by experiences, and the way one sees things.
The desire for harmony: For many people, living in a harmonious environment, is crucial. Especially in cases when someone grew up in a family, community, or country that is marked by violence, the desire for harmony might be healthy (or the opposite!). However, reflecting why harmony or destabilizing harmony has a high priority remains unknown for many. 
Understanding: For some people, it is essential to feel that the surrounding understands one’s viewpoint and reasons for acting. The reason for that can be the feeling of being misunderstood in the childhood way too often, a mother with a mental disorder, or low self-esteem. However, becoming mindful of why this desire exists or lacks can be especially crucial during mediation.