The problem is that even when you’re right, it’s coming from the person who always dissents, so people will listen to you less.
It’s that you’re usually advocating holding off, holding back, or not starting.
It’s that, in fact, things usually are not right in the first place, but they get right once they’re in motion, not when they’re stuck on the drawing board.
But the worst part is that it keeps you safe. You don’t experience the fear of maybe failing. You don’t discover that things that are less planned, less orchestrated, less thought through sometimes work spectacularly. You don’t learn when it’s time to say “hold on” and when it’s time to go. Both have their moments.
(And of course no one thinks of themselves as being a naysayer, they’re just offering constructive criticism…)