Be careful who you are associating your life with. Some people will manipulate you for their own selfish advantage. Some people are only good if they can benefit something from you, they will pretend that they are kind, they will pretend that they care because just like I said... they can get something from you.
But once you can't give them something anymore... their true colors will be revealed, they will show their true attitude and you will feel bad because you feel that you've been manipulated and used. You will feel that you've been treated unfairly. It is a disgusting feeling because you will feel dumb, stupid and unintelligent. You will feel that you've been betrayed and you will begin to doubt your confidence.
So start reading people's behaviors, start being cautious if they are using you or not so that you will not feel bad in the end. Don't be an accessory to someone's success. Don't get manipulated by people who is not even close to you or doesn't have any special place in your heart. Always learn to say no when needed, never do something a favor if you think that it will not help you to become successful.
Be aware if someone has a hidden agenda if he is trying to be close to you. It is not about thinking negatively, you're just protecting yourself from users and manipulators.