Most Helpful Girl
Because they weren't always treated in a mean way. Usually during the initial stages of dating or getting to know someone, people put their best side forward. lf they treated you like shit early on you'd never have contact again with them. You wouldn't be drawn to them. It would be easy to walk away, because you wouldn't have develops feelings for them , or an emotional connection with them. As times goes by, you start to develop feelings and an emotional connection with them. Gradually, over time, they seem to change. Their true character starts to show. So when they treat you unkind , you cling to the happier times, and the person they "used" to be. You hope they'll change back to how they were at the beginning. Not realising who they "were" is not who they really are.
Most Helpful Guy
Because you give them a part of yourself and they have rejected a part of you, so it is harder to get over. If you're open with someone and they put up down, we tend to think we did something, or it was our fault or whatever. The people were most vulnerable with create the biggest wounds or heal us, but we don't know until we put the effort in and they reciprocate or reject us.