Easier said than done, if you know what I mean. If it were so easy to think positively then people would no longer be diagnosed with depressions, need therapists etc.
If the people diagnosed with depressions would all follow this easy and simple rule(to love yourself first and foremost, to think positive) it would be like that....when i am down(and i am a lot, believe me) i am forcing myself to think positive, go back to the roots of myself and it works...sometimes you have just to think about worse situations than yourself are in...i am not downplaying depressions, being myself depressed for a long time in the past(what developed into a nervous breakdown what lasted over 2 years and i helped myself out of that mess with just talking about it to everybody)i believe Depressions are a serious condition but rather than just prescribe tons of pills, the shrinks (sorry therapists)

need to provide real help, lend an ear etc.
Anyway you are right, life is not all joy, to be honest life almost sucks all the time but if you try to have a positive attitude about it, you can conquer it way better
And your comment about"pessimism or optimism are attitudes that are wired in our brain" is in my book just wrong,as a hobby shrink i predict you have had a bad childhood, it all boils down to a bad childhood lol...but on a more serious point of view on this..the things hot wired to our brains are:
Male:
Reproduce,Reproduce,Reproduce(go Tiger lol )
Hunt, Hunt, Hunt
Eat Meat to be strong so that you can provide for your family
Reproduce,Reproduce,Reproduce
Female:
Give Birth
Cook dinner
Clean the cave and make it cozy
(sorry girls hehehehe)
Sounds strange? yes but these are the programmed things in our genes, only the ongoing evolution is shifting things a bit but there is no wired thing in our mind like pessimism or optimism, it is all about how you learned to live your life, either from your parents or your peers or your environment.
Let me explain what i mean by that....almost everybody is afraid of spiders, at least i was since i was a little kid...couldn't stand those little critters, killed the smallest, killed the biggest whenever i saw them....because i was afraid of em...but i was always curious about them, what was the purpose of having 8 legs, being hairy, ugly but rather useless in my opinion?? ...still afraid(hey i was jumping higher than a girl when i saw one!)
And one day i thought"hmmm, why are you such a pussy when it comes to spiders?" and i started to research things, facts....Make a long story short, i ended up to buy myself a "Tarantula"!... man was i afraid of this HUGE critter...but this critter ended up crawling,walking on my hand and it was the most exciting feeling i had to this day,(well only one other was so exciting,having Sex with Megan Fox) lol...but it was that feeling, it was so soft, so gentle, i never ever in my life thought that a big friggin Tarantula would walk on my hand

(i ended up to breed them, having 7 different species, it was quite lucrative i have to say since it is a niche)
So, why am i telling you and everybody this, you ask?
Depressions are build up on fear, the fear of rejection, the fear of losing, the fear of failing, the fear of not being successful even the fear of not being as good as your brother, sister, family member(here i go again, it boils all down to the childhood)
But you can overcome this fear, just believe in yourself and think a bit more positive, try it!
Alright everybody, i wish you all the best, stay positive!!
Cheers
Liquid