Because a ladder has rungs. That is how it works. We all get stood on by somebody. Join the human club my friend.
Perhaps because you have a big heart, and perhaps because you believe in redemption. Maybe also because you are willing to be friends with someone if there is any chance at all, and you can't walk away if there is any hope left?
It is hard to learn when to say it is enough, though it gets easier with age and as you begin to realize that some people will not change, and that you aren't responsible if they don't. Your tender heart will get a little harder, but that seems to be part of life.
from a famous poem:
As far as possible, without surrender,
be on good terms with all persons.
Speak your truth quietly and clearly;
and listen to others,
even to the dull and the ignorant;
they too have their story.
Most do it for approval, affection or what passes for love.
I wish I knew. The psychologists would say b/c we let them walk all over us. It's a difficult habit to change. People around us don't like it when we change and become more assertive. They have got used to being able to walk all over us.
Someone is doing just that to me walking all over me, has been for a long time now ! but I also know one day I will get out of this, they are winning the battle, but they won't win the war, thing's change !