These happen because of something the participants can't control. An example is spouses who work different shifts and therefor don't see each other very often. This can lead to tension and ultimately conflict.

Sometimes people will have a conflict because they have a fundamental disagreement on fundamental values. For example, some people may value hard work above all else, while another person thinks it's wrong to waste your life on backbreaking labor.

This is when two people fight because they both want something, but there aren't enough resources for both of them to satisfy their desires. The classic example is when people fight over money.

Sometimes people come into conflict because they are jockeying for position. When you have two people who believe they should be in charge, conflict can easily erupt. A classic example would be the parent fighting with a rebellious teen.

Sometimes misdeeds between two individuals can build up over time, and people eventually become bitter. These pent up feelings can erupt and lead to conflict. An example would be a couple who constantly fight over a history of infidelity.

There are cases where two people have totally incompatible personalities, and they will find each other difficult to tolerate. These kinds of incompatibilities can lead to conflict, especially if the people are forced to spend a lot of time together. For example, if you have one person who loves to talk, and another who hates talking while they work.

People often have conflicts because they have a different interpretation of the facts in a situation. An example would be two religious leaders with different interpretations of a sacred text.