At the end of the day, expectations are made in both a friendship and relationship. We expect people to do the right thing, we expect people to be there for you etc, but what happens when people just let you down? Constantly letting you down over and over again. You're just left with disappointment and to be honest with you, it's not the greatest feeling.
Have you ever felt like you've cared about a certain person and would do anything for them yet they pretty much take you for granted? It sucks, doesn't it? When you're constantly putting so much effort in a certain relationship and getting nothing back. What's the point really? There really isn't any point if you're wasting energy on certain people that don't give a shit about you, that clearly don't want anything to do with you and that feed you with bullshit and shit excuses.
For a while now, I've felt like I'm a door mat to some people. Like they can just freely walk all over me whenever they please but when I ask something from them, just a simple favour, they do nothing. It's fucking great. Note the sarcasm. Either way, I can tell you now that people are going to disappoint you.
No matter how close you are, no matter how 'good' your relationship may be, people are just going to disappoint you no matter what. It honestly can be anybody and it'll be most likely the person you'll least expect it to be.
