I am an example of an optimist-dreamer-turned-pessimist when I became a JW.
You just slowly forget dreaming about a good future (in this lifetime). In essence, there is really nothing to look forward to, except, that wonderful "new world".
I studied in one of the top universities and made good grades and did well---but after becoming a JW, I couldn't be "properly proud" about my achievements. I did not "sell" myself well because I lost ambition.
Bottomline is: humans sometimes need to share their achievements and celebrate them. But in the JW world, you can only be proud if you have a Bible study, or if your BS got baptized, or if you are given more "privileges". Of course, not everyone becomes a pioneer and so on and so forth.
How would you feel if you cannot dream big, cannot share and celebrate your achievements of the few things you can do well in this world??