There are those who want to tell you that "can't" is a four-letter word, in other words, a naughty curse-word. Apparently, this is an attempt at motivation to be optimistic by pointing out that pessimism causes swearing. Evidently, pessimism is not obviously bad, but swearing is. Who can tell what the hell these people are trying to say?
The truth is, "can't" is a five-character word, the same number of characters in the word "bacon." If you're not sold on that alone, it must be noted that "can't" is a contraction for "cannot." This has six letters, the same number as in "better," "pizzas," and "sexual." If you were dumb enough to believe the four-letter word argument about "can't," you must now change your opinion based on this new unrelated and irrelevant information.
"Can't" is one of the finest words in the English language. Nothing so simple ever got so many people out of so many undesirable situations. Where people are offended at "I don't want to," they are sympathetic at "I can't."
When you don't want to go to work, you still have to go. When you can't, however, there's nothing that can be done!
In addition, without the marvelous word "can't," there is an abundance of sage advice that would be more difficult to give. "You can't bring African bullfrogs into the country," "you can't drink Clorox and live," and of course, "religious wing-nuts with degrees from uncredited Christian universities can't intelligently comment on well-established sciences like evolution."
Remember, if you always tell yourself "I can," or even "I think I can," you are if for a lot of unnecessary work and trouble. The little engine that could may have gotten lucky that one time, but next time he hits ad even higher hill, what's going to happen? His false confidence inspired by his brainwashed mantra "I think I can" will cause him to try to conquer hills out of his league. Two-thirds of the way up one of these hills, the mechanical stress will finally tear apart his little moving parts, and he will uncontrollably roll back down at great acceleration, slamming into the freight train 10 minutes behind him on the track at over 100 miles per hour. His broken metal husk will be scrapped, melted down, and made into cheap folding chairs that will be sold to the Church of Scientology.
See what optimism gets you? Nothing but Scientologist butts for the rest of your existence.
We need "can't." We can't do without "can't." Someday, you'll look up a metaphorical mountain, assess your situation realistically, and then tell yourself "I think I can't."
So every day, with each task you try to take on, take just a moment to think: "what if I can't?"