Why do we settle?

Okay so I understand that it's in our nature as human beings to have someone... For the simple fact that we don't do well alone... (Example: Britney Spears) 

But really... Must we settle for crappy ass people...?

I mean... Is being alone so bad that we will knowingly get with someone or hang around someone who is the epitomy of a shithead... And allow them to make us the fool?!?!?

Seriously though... We will straight up ignore signs that someone is not genuine and is knowingly messing with our emotions... And believe the act that they are innocent... 

I mean I can't imagine that if someone is causing you pain... Or making you feel uncomfortable... That they have no clue that they are doing so... I just refuse to plain and simple...

Why is it so hard for us to say what we feel and risk losing the shithead who's causing you grief?Why is it so easy for us to settle for mistreatment by someone who claims to care about you? 

I just don't get it....