2 flaws of capitalism
A barometer for any economic system's success is it's ability to increase happiness. Capitalism has been a successful system at increasing happiness, though it does come with flaws in it's purest form such as allowing monopolies and allowing companies to stifle competition. These types of problems are part of the two biggest problems in the most purest forms of capitalism.

Only thinking of your own interests

This is normally a benefit thinking of your own interests as it improves competition, but sometimes you are not the only person at risk from the actions you're taking. Imagine we allow companies to freely buy land, build a nuclear power station and have their own safety regulations. As a owner of the power station your risk is the profits of the power station, not of people who live around the power station. As you can see risk isn't just on the owner of the company and the people who buy the products.

Elk need larger antlers than their peers when fighting to find a mate, but the larger antlers actually make it harder to run away from predators, it would be in all their own interest if they all had smaller antlers as it doesn't matter how big the antlers are but how big they are to your nearest competitor.
This is the same case for certain companies when advertising, you have to spend more than your competitor on advertising but this hurts both your companies having to spend so much money on advertising, it would be in both your own interests to stop advertising in certain cases.

People not knowing what's in their best interest
There are two main reasons people make decisions that are not in their interest's. 

1. Lack of knowledge can make it hard to compare two products or to make a decision on if a product is right for you harder. To know if you are getting the best and appropriate healthcare can be difficult when you don't really know how to even compare what is a better healthcare plan. You may believe you have all the relevant information to make a decision but even then you can't be sure and it's in the companies interest to suppress information that would be harmful to it's profits.
2. We are not 100% rational beings, our brain's are wired up to want and act certain ways what can be detrimental for us and against our own interests, such as wanting to loose weight but having craving to eat more, wanting to stop smoking, etc. capitalism can play on our craving and the quirks of how our brain is wired up and actually make us more unhappy in the long run.