Perhaps it starts with a simple favour, like loaning a friend five dollars or helping with child care occasionally. Before you know it, you are covering for that friend at work and cosigning for her car loan, all at your own peril. People who take advantage of other people's kindness will always be around, and it's difficult to stand up to a mooch when she has already targeted you as susceptible to her charm.
Who Takes Advantage
It's possible the person is just lazy and doesn't realise he is using your kindness to skirt his own responsibilities. However, if you are being taken advantage of unfairly, it's possible you are dealing with a narcissist. The narcissist believes she is the most important person in the world and does not care about your feelings, which is why she takes advantage of you. In her mind, your needs are dramatically less important than hers because of her imagined superiority, so she doesn't see it as taking advantage at all. She feels entitled to your kindness because she is better than you and everyone else, as far as she is concerned.
Worse, you could be dealing with a sociopath. A sociopath has no empathy or conscience and only worries about his own happiness. He completely disregards the rules, right and wrong, and your needs. He will thrive when taking advantage of your kindness, and the sociopath may go so far as to cause you harm to get what he wants.
Why They Do It
A normally lazy person will take advantage of you because of the convenience; if you will do something, that means she doesn't have to do it. A narcissist feels she is the most important person in the universe, so it is only natural that you exist to serve her, and she will expect you to do as she wishes. A sociopath's goal is to get everything out of you that he can, and since he has no conscience and no shame, he is not concerned about taking advantage of you.
How to Stop Them
It is important to be assertive with people who don't appreciate your kindness. Express yourself properly and clearly by talking about how you feel and avoiding accusatory words. Don't use "uh," "um," or "I mean," because these phrases take authority away from what you are saying. Don't allow yourself to be charmed. If you can find the courage to stand up for yourself, it will show the person that you do not exist for her convenience, and she may change her behaviour. If the person is a narcissist or a sociopath, he will eventually realise he can no longer mooch off of your good nature and will move on to greener pastures.
How to Avoid Future Mooches
It is important to guard yourself against predators. What begins as an annoying mooch can easily turn into a dangerous situation. Lazy people let you pay for dinner; sociopaths steal your identity. Both do these things because they can. Keep your information safe, and don't do favours for someone until you've got to know her well. It may feel strange at first, but in the end, you'll be protected. Knowing that your good deeds are saved for those who can truly benefit from your kindness will make it worth your while.