The Pollyanna Principal is the tendency for individuals to remember previous pleasure as being much more significant than previous pain.
For example, I can remember getting a good score on a test that happened three months ago, but it is difficult for me to be able to visualize the headache I had a day ago.
Suffering seems to be a much more potent feeling than pleasure, except when it comes to memory.
It is my own personal theory that since we base so much of our decisions and lives on memories, and we tend to focus much more intensely on the pleasurable aspects of our past, that this gives us a kind of motivation to keep going, to expect the future will also be like this.
This stems from a general observation that I've had concerning how people evaluate their lives; most people think their lives are alright, and I've been wondering why this is the case and why they don't realize their lives aren't as great as they think they are. I'm wondering if Pollyanneanism is some kind of drug that masks reality and allows people to be happy, considering substantial scientific research has shown that people who think this way take longer to realize negative things in their lives, and "overestimate" the positive.