The chances are that during your lifetime, you will encounter people who always seem to let you down. You may lend them money that they never bother to repay; ask them to do something for you, which they never get around to doing; and it often seems that you're always giving without receiving anything in return. Sometimes, you're better off just cutting these people out of your life, but if they are a close friend or relative, it can be hard to just let go of them, even if they do treat you badly and always let you down.
Often, when someone lets you down, it is not a deliberately malicious act; they're just too wrapped up in themselves and their own lives to really pay attention to what other people are thinking or doing. They make rash promises that they fail to keep, whether intentionally or not, and yet still expect their friends and family to drop everything for them. They are so self-absorbed that they don't bother to think how their actions affect others around them. As long as they are having a good time, they don't really care and they try to avoid as much responsibility as possible.
Of course, it doesn't help if you continually indulge these individuals. If you have a friend who frequently asks to borrow money from you and rarely pays you back, you can hardly expect him to learn anything if you always give in and hand over the cash. It can be tough to say 'no', especially when the type of person who always lets you down usually has a tendency to be manipulative and skillful in getting exactly what they want. If your 'friend' starts talking about how he has nothing in the house to eat and won't be able to feed his cats, the chances are you're going to feel sorry for him.
However, you're the one who ends up feeling frustrated, upset and angry when you lend your friend some money and he never bothers to pay you back, especially when you know that he's been down the pub and out clubbing with friends. It hurts even more when your supposed friend doesn't even bother to make an effort to see you, unless he wants something from you. It is hardly friendship if you are the one who is always supportive and willing to give up your time and money for someone who doesn't appear to care at all what is going on in your life.
A relationship in which you are always being let down is hardly beneficial to you or the other individual concerned. You're left feeling used and the other person thinks that he can keep getting away with treating you like dirt. You therefore have to face up to the fact that your own well-being should be a priority and that if you have to cut that person from your life to be happy, then so be it.