Why does he treat me like I'm his girlfriend, but won't actually be my boyfriend?

We've been best friends for like 7 years. Everyone always asks if we're together because we hold hands everywhere, he's always kissing me, and we take pictures kind of like this.
Why does he treat me like I'm his girlfriend, but won't actually be my boyfriend?
Whenever I asked him if we were ever going to be official, he accused me of making things awkward and to just let things be. But if I start talking to another guy he gets pissed off and ignores me. I ask him what I'm supposed to do because I don't want to be single forever and he just says that "I'm his" but we don't have to be together for that to be true. He makes zero sense and I'm getting annoyed with him.
Why does he treat me like I'm his girlfriend, but won't actually be my boyfriend?
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