Why should you not overuse words like 'believe' and 'think'?

When you're writing fiction, using believe or think means you're telling your reader something instead of showing. Consider the following examples:

She couldn't believe her eyes.

She stumbled to a stop, gaping at the sight.

In the first example, you're telling your reader what the character is feeling. In the second example, you're showing astonishment through the character's actions.

When you're writing non-fiction, believe or think turn the focus on the writer instead of the subject of the sentence. You don't want to draw attention to yourself as the writer, so drop the believe and think and make a bold statement.

I think the candidate has a pertinent point on environmental issues.

The candidate has a pertinent point on environmental issues.

The first sentence is discussing what you, as the writer, think. The second sentence makes a bold statement that focuses on the candidate.