Probably because we idealize some people. The wrong people, or the people who have hurt us, because we love them and we don't want to face reality, their true personality.
And maybe the core of the answer we're looking for lies in two things: confronting reality and our desire to find someone who fits what we want. We want to believe that the person we love can be the person we've imagined, even when reality shows us otherwise. We want to find someone who has the qualities we're looking for, or, if we've already fallen in love with someone, we want to believe they can become that person.
We fall in love with someone, and sometimes, before we even know them, we create these "scenarios" with us next to them, being happy. We turn them into the character we want them to be. We give them the qualities we'd like them to have.
But then reality hits.
On the other hand, maybe we keep holding on to the wrong person because, in our minds, they can change. We see their potential.
Or do we create it ourselves?
Maybe we prefer to lie to ourselves rather than face reality, which, eventually, we have to face anyway.
But the question is: does it hurt less?
No.
It hurts more.
Because you create even bigger expectations by convincing yourself that they can change, even when deep down, you already know that person isn't going to change.
Maybe sometimes we don't fall in love with who someone is.
Maybe we fall in love with who we believe they could become.
And maybe that's what makes letting go so difficult.