Being optimistic is fine. Being pessimistic is also fine.
Too much of either is too much.
Truth be told: Too much of anything is too much.
You can' live your life in perpetual joy as you can't live your life in perpetual sadness. Even if in the distant future we'd be able to have constant artificial joy worlds, it would be just that: fake.
Think back of the last time you were in line, waiting for a big event. You were excited even before it started. Those kinds of events (let it be a music show or a conference) are designed to make you excited early on. The boredom of being in a line lessen if you can't stop thinking about what's going to wait for you on the other side.
I act like nothing happens until it happened cause I don't want to be disappointed, my wife is the total opposite. She acts like it already had a good outcome even if it didn't happen yet. Both behaviors are not ideal.
The first never makes you excited for what's going to happen (I still get excited about stuff, but it must be really out of the ordinary to have that effect). The second set you up for disappointment if it doesn't actually happen in the exact way you imagined it.