Self-aware people recognize that they sometimes act irrationally. So how can self-aware economists assume that other people act rationally? Here's how David D. Friedman answered that question in his 1996 book Hidden Order: The Economics of Everyday Life (pp. 4-5).
"Suppose someone is rational only half the time. Since there is generally one right way of doing things and many wrong ways, the rational behavior can be predicted but the irrational cannot. If we assume he is rational, we predict his behavior accurately about half the time--far from perfect, but a lot better than nothing. If I could do that well at the racetrack I would be a very rich man. 
“One summer, a colleague asked me why I had not bought a parking permit. I replied that not having a convenient place to park made me more likely to ride my bike. He accused me of inconsistency. As a believer in rationality, I should be able to make the correct choice between sloth and exercise without first rigging the game. My response was that rationality is an assumption I make about other people. I know myself well enough to allow for the consequences of my own irrationality. But for the vast mass of my fellow humans, about whom I know very little, rationality is the best predictive assumption available.
"One reason to assume rationality is that it predicts behavior better than any alternative assumption. Another is that, when predicting a market or a mob, what matters is not the behavior of individuals but the summed behavior of many. If irrational behavior is random, its effects may cancel out. 
"A third reason is that we are often dealing not with a random set of people but with people selected for the particular role they are playing. If firms picked CEOs at random, Bill Gates would still be a programmer and Microsoft would have done a much worse job than it did of maximizing its profits. But people who do not want to maximize profits or do not know how are unlikely to get the job. If they do get it, perhaps through an accident of inheritance, they are unlikely to keep it. If they do keep it, their companies are likely to go on a downhill slide. So the people who run companies can be safely assumed to know what they are doing--generally and on average. And since businesses that lose money eventually shut down, the assumption of rational profit maximization turns out to be a pretty good way of explaining and predicting the behavior of firms."