Your point is well taken. I'd say there's a few arguments against unbridled optimism, but the importance of those varies from person to person (and season to season)

1. Respect from others.  If you expected BYU to win the CFP last year, a lot of people will start giving your opinions less credence. If you expect them to definitely win this year (since your prediction for last year didn't come through), people are less likely to hear/respect your opinions.  Both on that topic and on others.  As mentioned, mileage may vary on how much this matters to you 
2. Cynicism. If you expect BYU to win the NCAA tournament and the CFP every year, you're going to get let down a lot. And for most of us as human beings, it becomes impossible to keep expecting greatness if we've repeatedly been smacked in the face by a reality that doesn't mirror it.  After a point, it's hard to keep unrealistic hopes alive. So tempering expectations can be a survival mechanism to keep the ability to hope alive.  Again, you might be the exception with the unlimited capacity for hope, but most aren't.
3. Joy in results. The argument you make is that a good season always brings joy and a bad season always brings disappointment.  But good and bad are relative. BYU and Kansas has very similar basketball seasons last year, but due to differing expectations, BYU's was a joyful triumph, while Kansas' was a bitter disappointment.  An 8-4 football season would be an exciting joyful success for many (most) this year.  But if you expect BYU to be Ohio St this year, then 8-4 is still a disappointment 
4. Joy in expectations. This basketball off-season has been a blast precisely because it is out of the norm, and raises excitement and expectations. If we really had the expectation every year of being #1, then we could never take joy in an enjoyable off-season that raises expectations...because they're always already there.  Without the sadness, there can be no joy

Note that none of this is to say we should ALWAYS downplay expectations. The variety of expectations is a huge part. And this year absolutely has more reason for high expectations.  But it's also important to temper them to some extent. If you come into the season with a "championship or bust" mindset (which most are not, but just as an example), then you're much more likely to be disappointed.