How we think influences how we feel. Well, to be more precise, how fast we are thinking influences how we feel.
In a series of studies, Emily Pronin (psychology professor, Princeton University) and colleagues have tested the effect of thought speed on mood. In these studies, thought speed is increased (manipulated) in one of several ways, such as how quickly people are told to read or the speed at which a video is being watched. Participants then report their mood at the end of the study.
Sometimes, the content they are reading is negative and sometimes it is positive. But regardless, mood is more positive under conditions in which thought speed has been heightened. In other studies, thought speed has increased creativity, self-esteem and general feelings of energy.
This is consistent with research among people with mood disorders which shows that depression is generally marked by a slowness of thought, whereas mania—and the resulting euphoric feelings—involve a particularly high level of thought speed.
In general then, thought speed appears to be asociated with heightened positive affect. One exception to this might be when fast thinking is repetitive. When thinking the same (aversive) thing over and over, anxiety may develop. This is consistent with research on rumination, in which repititive negative thinking results in heightened negative affect. Conversely, thought variation at a high level of thought speed is particularly likely to result in high positive affect.
In summary, it is not just the content of our thoughts that influences our mood, but the speed at which we are thinking those thoughts. Fast thoughts are most often associated with feeling good.