People forging a relationship are optimistic that it will last a long time. If they succeed, they often find great comfort in being with the same person for years. They learn the other person's wants, needs and peculiarities. This gives them a feeling of great knowledge because they have learned what makes the other person tick. They can tell, without guessing, what that person will want for dinner or for a birthday gift.
Getting comfortable in a long term relationship can be dangerous. People begin to believe they know everything about the other person. They make assumptions based on past experience. Often they are right, and this enhances the relationship. Occasionally, they are wrong and it causes problems. When two people are together for years, complacency is the biggest problem they will face. Being wrong about the other person is a wakeup call that the relationship has stagnated.