Optimism comes from within. My point exactly is that, you don't actually need another person to be positive in life. Most of the time, our negativity is from our anxieties and or from our bad experiences in the past.
So is it irreversible? Of course, it will be up to you on you can hack your life and see the good things around you.
Some of us are having difficulty appreciating even the simplest form of positivity ~ sunrise. Some would say, it's just a simple sunrise and it has nothing to do with me. It won't even affect me. I personally don't think of that response to be sensible.
If were that person I'd rather appreciate the warmth of morning sun and the bright day ahead. The morning sun would also help to sanitize my clothes and the interior of my room if there's a sunshine going through my room. As simple as that!
It is simple to teach possitivity to a person, but it will take time & initiative for a "pessimistic" person to flip and be more appreciative about life.
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Yes ... You are true, motivation and happiness should come from their own hearts and thoughts... Nice way to start a day with