There are two different ways to view time. One is the linear view, and the other is the cyclical view. The different views will affect one’s attitude. The linear view is where time has a beginning, a middle, and an end. The cyclical view is where time has no beginning and no end. Time starts in chaos, grows to order, and then declines back to chaos, and then the order repeats.
After Christ, more people started believing in the linear view than in the cyclical view, and that is when people started moving forward. Since people started thinking differently, they started progressing. They no longer thought their progress would just regress back to where they started.
As one can see, people who believed in the linear view were more optimistic because they thought they would keep progressing in the future. People who believed in the cyclical view were more pessimistic because why progress when they are just going to regress back to where they started.