[You are equating thinking that it will rain, if you say it won't rain with negative thinking. That's more than a little silly.]
It's about focusing on what you want, not what you don't want. Whatever you focus on will manifest in some way or emotion, so it's best to stay positive, rather than focusing on what you don't want to influence your life. Kind of comes down to the confirmation bias.
Not that anyone will take this seriously, but I still remember the day my dad told me rake the leaves from the front lawn when I got home from school. I didn't want to, so started watching TV after school. I actually prayed which I don't normally do that I wouldn't have to rake the leaves. In the next 30 minutes the wind really picked up and blew all the leaves off the front lawn.
I'm not saying you can control the weather, but a positive attitude goes a long way to achieving your goals. You will be less likely to become discouraged and give up.