Our expectations can alter how we view reality as a whole. One time, at a party, I picked up what I thought was my can of beer and took a sip, and was revolted by the flavor.  It turns out I had picked up a perfectly good can of Coca Cola, but my mind, expecting the taste of beer, was disgusted by the flavor of Coke.  Once I realized what it was, I took another sip and it tasted fine, but my expectation had altered by experience.

More from the interesting Lifehacker article:

The way we anticipate something changes the way we perceive it... It suggests that the way the brain works is to influence our perception. If our perception has been established without the information from the brain, the information of the brain is no longer relevant... 

If you think about it more generally, there's a question about how our preconceived notions color our view of reality... what happens when we view the world with glasses that are strongly tinted by our preconceived notions? What these results suggest is an interesting connection between the body and the mind... it suggests our mind tries to predict the future... by anticipating the future the mind actually changes our physiology... it prepares us for that future. By doing so the mind basically gets us to experience the reality that we anticipate. 

That was from psychology and behavioral economics professor Dan Ariely,  His concept falls in line with the idea of the hedonic treadmill. The hedonic treadmill is the tendency of humans to quickly return to a relatively stable level of happiness despite major positive or negative events or life changes. According to this theory, as a person makes more money, expectations and desires rise in tandem, which results in no permanent gain in happiness. The pursuit of happiness can be compared to a person on a treadmill who has to keep walking just to stay in the same place. 

The concept dates back to such writers as St. Augustine, cited in Robert Burton's Anatomy of Melancholy: "A true saying it is, Desire hath no rest, is infinite in itself, endless, and as one calls it, a perpetual rack, or horse-mill."

Basically, if a B+ made you happy last year, it'll take an A- to register the same satisfaction again. If you think you'll do poorly on a test or in a social situation, you probably will .  You've probably heard the classic phrase "happiness equals reality minus expectations," and it's true. In short: you can theoretically apply the placebo effect to your day-to-day life. 

Science journalist and author Chris Berdik offers up this example of how exactly this works:

Many people worry that they're likely to choke under pressure. They look to coaches and elaborate training techniques to overcome this tendency. Or they just worry and bite their nails before important presentations or competitions. But in one study, researchers told some track athletes that what they thought of as pre-race jitters actually improved performance, while telling another group that this sort of arousal was usually detrimental. The athletes performed accordingly when the pressure was on. In another athletics study, the researchers gave every subject a personality questionnaire and then randomly gave some of them false feedback that their answers indicated they were the sort of person who thrives under pressure. When it came time to compete, the athletes told they would likely do better under pressure did so. 

Of course, this doesn't mean you can change the world around you with your mind. If you're sad, you'll still be sad, and if you're ill, you'll still feel sick.   But what it does suggest is that we're more in control of our present and our future than we think. When we fully realize this, that it's our own mind that causing us to suffer form sadness or from sickness, the sadness and the sickness lose a little of their grip on us, making it that much easier to get better.