Being hopeful is what fandom is all about.
Being a homer is necessary to root for a team.
If you are always pessimistic and never believe that you might exceed expectations, you're not a fan.
I'm always slightly optimistic for every game and I know WHO WE ARE. (which gives me even more credibility)
I'd say I'm 1% more optimistic than pessimistic (optimism always ultimately wins out) and that is all it takes to be a fan. If you are any less than that, you are, at best, a bandwagoner. (cireksu, rustyksumichiganwhatever, other famous pessimists)
Pessimism is meant for the long term/big picture.
For example,
Kansas State will never win a National Championship. I know that is absolutely true. But fandom makes me believe that maybe, just maybe, on the one millionth attempt, we might actually win it.