It is generally accepted that increased income levels result in increased happiness for individuals. An economist or politician who subscribed to such a premise would advise policies aimed at increasing income levels in order to improve the subjective well-being of a person or group.However, surveys worldwide consistently find that self-reported happiness is only positively correlated with income up to certain(and rather low) point, after which increased income levels exhibit no correlation with happiness levels. It is possible that the satisfaction of basic needs is the only material contributor to human happiness, while all other relevant factors are intangible. But what if another explanation exists: what if happiness depends in part on self-perceived income relative to one's aspiration.

What does the highlighted line mean?