Answer:
If you are aware of positive thinking you are conscious of positive things. You are thus more concious of the positive situations around you. This has the effect of 'attracting' positive situations around you.
If you give to someone , they are more likely to give back. So the more positivity you give in your life, the more you are going to get back, spiritually, mentally and socially.
Thoughts can be measured by a frequency. If you were hooked up to a machine and thought about different things they would all give off different frequencies. Frequencies have the ability to attract other like frequencies. Thus thinking positive attracts the same frequency thoughts to you.
Finally, everything in our lives is what it is because of how we create it. We are sad about something because we create that sadness. We are happy about something because we create, essentially choose to be happy about something. This creation and choosing can occur on a concious and subconcious level. We may argue that when we feel sad about something that happens to us it is a natural reaction. This is true, but it is a reaction because of how we have chosen to deal with situations like this in our lives.(This is greatly affected by our growing up, back ground, religious beliefs, relationships etc) If you experience a car accident personally it can be VERY traumatic, if you hear about one on the news you often do not think about it again. To attract 'happy' and 'positive' situations to you become more concious about your thoughts. Choose to feel happy as much as possible in your different situations. Becoming concious about all our thoughts is very difficult at first because we are trained otherwise. The more practice you get the easier it is. Thinking something at first is just a thought which does not have much power to attract. The more you think it, the more you start to believe it and the more power that thought has in your life. It then becomes easier to be concious about your thoughts.
There is a huge amount of literature on this topic. A good starting point, Neale Donald Walshe: Conversations With God.