living for herselfAfter Kyouko felt like her wish destroyed her whole family, she swore to herself she would never use magic again for anyone else but herself. She realized that miracles aren't free. When she wishes for something good, an equal amount of bad or evil is placed in the world to balance it out. Everything cancels out to zero in the end in order to maintain the balance of the world. Kyouko believes that it's easier to let go of all your worries and focus on yourself. She believes that she has already paid too big of a price for what she got while becoming a magical girl.
Because Kyouko has this type of mindset, she is able to accomplish a lot more. She is able to get many grief seeds in a short period of time, and constantly develop her strength. She blocked her mind out from other people's issues, and became as selfish as she possibly could be. She felt like keeping to herself and only living for herself would limit the pain others could make her feel. If she constantly made her own life decisions, she can't blame pain or despair on anyone else around her. She honestly believes that living for herself is the only way to get through anything. It's hard to regret anything if you accept what you are doing is for your own good. Once again, this is why she has great mental strength and is able to keep her soul gem pure.
I think it is important to note the difference between being selfish and being self-centered. Being selfish means they have a strong bias towards not giving anything to anyone other than him or herself. This includes time, money, effort, energy, support, etc. While a self-centered person can care more about ones self, but they still have morals of compassion and generosity. Kyouko is definitely self-centered, but not selfish. She still gives to people who she trusts and cares for like Sayaka. We also often see her offering food away to people as a form of compensation or to say hello. Food is very important to her because she knows what it is like to not have it.