Why is my mom convinced that my relationship with my boyfriend won't last and isn't that serious, even though we've been together for 1 year?

She’s thinks my boyfriend childhood friend is having a relationship by the way. It hurts me that she making assumptions about. When she hardly knows him. I’ve trusted boyfriend he always reassured me which awesome. I don’t why she has a problem us being together.
I want her to support our relationship.
Why is my mom convinced that my relationship with my boyfriend won't last and isn't that serious, even though we've been together for 1 year?
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