That is the natural course unfortunately. It doesn't "have" to be that way, but it presupposes strict self-control and shared morals/ethics. For example, it is in your immediate best interest to vote yourself favors/money, but ethically you shouldn't as it would be wrong to take from others or tell them how to live.

This standard, can persist, but it is such a high standard for the masses and breaking it can be done so privately and carried out through a 3rd party so that history has shown this standard always falls, without fail, given enough time. 

 Proper politics isn't to vote to "better" anyone, but to leave everyone alone to their own actions as much as possible. 

Voting in this way (to try to better some group) means you are forcing someone to do what they don't want to do, or rather, you know better how they should live their lives and spend their money than they do.

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