Forgiveness/Friendship

Why is it so easy for some people to forgive others and keep there mouth shut when something really does bother them and for others it is so hard? I think I am one that easily forgives and tries to forget but it doesn't always work. I have noticed that I sometimes let people walk all over me. I tend to make a friend and love them no matter how bad they treat me and they could be mean to me and make me hate them but then after a little bit of time I get over it. It is crazy to me how this works. It makes me mad to some extent that I let this happen but then again I know that I am supposed to forgive and forget. So I guess my question should be is there a limit to forgiveness? Is there a point that it turns into someone walking all over you and you not being forgiving?