The frame of reference within which you're working can be changed. If within the frame, you formulate as a postulate that "if someone gains, someone else loses," you will apply that belief to the situations that life presents, and that will be your experience. As some of the other posts have indicated, it's not necessarily a gain/lose proposition -- oftentimes what is perceived as a loss may be a blessing in disguise; what may be perceived as a win may also be a curse, for example, "I beat you to the ticket counter for the last boarding pass on a plane that crashes."
Consider re-framing life experiences as "What is in everyone's best interest?"
(The first thought I had upon hearing that was that "It's in everyone's best interest....to consider what's in everyone's best interest.....")
This does sound more like a philosophical or scientific question, than one related to real life. In science, one of the above posters noted Newton's conservation, but there is also the balancing of chemical equations. If you get into yin/yang types of issues, a plus in one area would be balance by a negative in another, at least as far as I understand it.
But I'm an optimist. When someone does a good thing, it benefits the person performing the act as well as the recipient(s). If I'm a bystander, I benefit as well, just from being glad that goodness exists. By the same token, an evil act diminishes everyone involved or watching.
Perhaps a play that exemplifies this theme is "The Merchant of Venice" by William Shakespeare in which Shylock is the stereotypical money lender whose greed causes terrible losses for others. However, as Portia presents the theme in her words, "The quality of mercy is not strained," the Christian qualities of mercy and compassion lie beneath the flesh.
So, we need look no further than the reports on the unconscionable money lenders in this country to recognize such mortgage corporations as Freddy Mac and Fanny Mae as winners since bonuses have been given to many of the CEOs of lending institutions while the losers are those "ordinary" people who have had their homes foreclosed. Often one can only rely on faith and the conviction that integrity is not worth the gold one receives for his/her soul that is sold to the devil. Sadly, in this time of materialism, it is only spiritually and ethically, then, that the greedy "come up short."
It's easy for many of us to feel as if there are many win-win situations in life, but look how we express even those things, "I gave up my full-time job." This does not mean to imply that pohnpei regrets it, but it was described as a loss. One of Newton's laws loosely paraphrased, "For every action there is an equal, opposite reaction." Look at things as they exist in nature-male/female; sun/moon; day/night; life/death. Perhaps what we should question is whether the loss is perceived as negative. Pohnpei gave up a job (lost) but gained so much more. How can that be a negative?
This reflects a very negative view of life -- that it is what you would call a "zero-sum" situation.
Clearly, this does the reflect real life in some situations. You could say that most competition is zero-sum. For example, if two people apply for a job and one gains, the other loses.
But there is so much of life that is not that way. Family life is, or should be, an example of that. I gave up my full-time job to take care of my kids. I hope that we have both gained from this, and I certainly feel like we have.