What makes people think that it is our obligation to forgive them?

Why would people hurt someone years later expect forgiveness my 39 year old sister called me a resenful and she belives that i should forgive her and actas if nothing happened that easily. My sister has always been the golden child the favorite along with my other siblings she didn't had to struggle her college tuition was paid for now she is struggling financially and expects me to forgiveand move forward. The thing is I really never fit in with my moms side of the family not sure why I was always mistreated now as an grown adult they expect me ti forgive them so easly. I thing is a dont haveto forgive them.

What makes people think that it is our obligation to forgive them?
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