I have to push back on this thought a little. It is my hypothesis based on observations and my own experiences that some people are just intrinsically predisposed to positivity or negativity to varying degrees. I am on the negative side. I have been ever since I was little. I didn't make a decision to be that way. I can't just make it go away by choosing it. I can fight it, and I do, but it is a lifelong struggle. Sometimes I am successful. Often I am not. I suspect many others are the same way.
I think it's fair to hope that people who have an easier time being positive have some awareness and consideration for those for whom it is not so easy. At the same time, people who KNOW they are negative need to try and avoid bringing others down with it, because that is not fair either.