Is it better to be more assertive or more easy going?

I had this "friend" who would do everything without warning or even asking me. Like messing with my food, saying we were going to hang out even though i didnt want to, and he would use my devices without asking me. He even tried to scan his finger so he could access my iPad. On several occasions I vigorously told him not to use my stuff without asking me and he got mad at me for telling him off. Then when I tried to be easy going i let him use my phone so that he could text his girlfriend because his phone was broken, only to discover that he was pretending to be me the whole time in an effort to keep her from breaking up with him.

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