Lordylor I don't agree that it either comes naturally or it doesn't. You can learn to be of a positive temperament. I did. I came from a very aggressive and negative childhood and was reared to sneer at and find fault with the world, to blame and feel self pity. Not surprisingly I was bloody hard work and boyfriends never stuck around. My only friends were as screwed up and ranting as I was.
Bit by bit I learned this was not effective. I read loads of self help books and acted 'as if' I was a cup half full person until I became, naturally a sickening Pollyannaish cup floweth over type. I still suffer from deep depressions (childhood legacy) and have been battling one recently - but when the illness doesn't have a hold, then it is perfectly possible to train yourself to look for the positive.
The biggest thing to learn if it doesn't come naturally is that while you can never control what happens to you, you can control your response to it. Even in disaster and tragedy some people rise above the situation to create solace, to find solutions for next time or to fight for justice etc. Others just wallow.
The other thing that works is to actively look for tiny things that you can appreciate at any time. I get massive pleasure from a good cup of coffee, cuddling the cat, flowers in next door's front garden etc. It works for turning round irritating situations too. Stuck in traffic? It won't move faster if you get furious, so may as well listen to good music or notice the sunset. Some people find this idea very irritating and it sounds sickly sweet written down, but if you practise it, it does increase happiness.