So the question is, "Is it realistic to be pessimistic?" Is it realistic to look at the downside of every situation? Is it realistic to try to see what's wrong, what won't work, and what can be criticized about people and circumstances? Is that a practical, effective, productive mode of thinking?
The answer is: Only when trying to decide on a risky venture. Otherwise, the answer is definitely no. Pessimism leads to depression, contributes to heart disease and cancer, kills joy, ruins marriages, and impairs one's ability to accomplish. Why? Because a person's point of view can influence reality, not in some mysterious way, but directly.
For example, I once knew a woman who said many times, "All men are pigs." This is a pessimistic stance. And because she believed it, she treated men like pigs. And they responded piggishly.
In many ways, whatever position you take will tend to become a self-fulfilling prophesy. If you take the pessimistic position that you can't do something, or that there are no opportunities available, or the competition is too stiff and you'll never make it, any of these can make you feel defeated. Those points of view can suck the motivation out of your limbs. And because you feel demoralized, you don't have a lot of energy and you don't try to find a way. The result is: You will never make it. Not because you can't, but because you are convinced you can't. Your point of view has altered reality, not in a mysterious or mystical way, but directly.
In other words, to be truly realistic, you'd have to take into account the entire real situation, which is that you and your opinion influence the world, and to take a pessimistic stance on something often makes it that way. In other words, pessimism is unnecessarily counterproductive and self-defeating. Habitual pessimism is unrealistic.
If you or someone you know thinks pessimistically, those thought-habits can be changed and it isn't difficult to do. If you think, "I could never change my thought-habits," that is your first pessimistic thought to change. Start here: Undemoralize Yourself.
Adam Khan is the author of Antivirus For Your Mind: How to Strengthen Your Persistence and Determination and Feel Good More Often and co-author with Klassy Evans of How to Change the Way You Look at Things (in Plain English). Follow his podcast, The Adam Bomb.