In Think and Grow Rich Chapter 16, the author said,
Do not expect troubles as they have a tendency not to disappoint.
Why do troubles "have a tendency not to disappoint"? Don't they "have a tendency to disappoint"?
And why would anyone expect troubles?
To paraphrase in general, it's an expression that talks about a self-fulfilling prophecy.
From Wikipedia:
Self-fulfilling prophecy refers to the socio-psychological phenomenon of someone "predicting" or expecting something, and this “prediction” or expectation comes true simply because one believes it will, and their resulting behaviors align to fulfil those beliefs. This suggests peoples' beliefs influence their actions. The principle behind this phenomenon is people create consequences regarding people or events, based on their previous knowledge toward that specific subject. Additionally, self-fulfilling prophecy is applicable to negative and positive outcomes.
While it applies to both positive and negative thoughts, the expression is more commonly understood when it comes to negative thoughts.
In short, the more you believe you will fail (or bad things will happen) the more likely it is that you'll inadvertently sabotage yourself (if only unconsciously), and you actually will fail. (So, your prediction will not fail—or disappoint—you in its accuracy.)
People can have a habit of predicting trouble if they see things (or focus only on things) that happen to them in a negative light. They talk about all the bad things that have happened to them, and then say that because of that they don't expect anything different in the future.
The opposite can also be true, but it's often not as apparent.
