Why do most have it in them to take advantage of sweet people?

My current boyfriend calls me princess and likes spoiling me on some dates. That doesn't give me the rights to take advantage of him nor others. My parents don't have it in them either to use sweet people.

However, many people use someone's kindness. My ex boyfriend was one of them. All it took was for me to tell him ''You're my A+; I'll do anything for you' and all of the sudden he acted arrogant, treating me poorly (all the way to asking for money and never paying it back) and really thought he was almost perfect. He even rubbed it in my face saying ''but you told me I was the best and you'll do anything''. I see many people with that mentality. Just because you're very sweet and didn't put boundaries (you thought they would have logic and be smart enough not to take advantage), they treat you like a doormat. Why?
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Doing anything for someone doesn't mean ''I'll treat you bad and do whatever I want and you'll tolerate me''. I see many people translating it that way. Nowadays I'm only sweet to those that are sweet to me. If someone is very sweet, I'll reciprocate. If someone is a jerk, I'll be a total biotch.
Why do most have it in them to take advantage of sweet people?
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