What to do to a friend who never serious?

I have a friend. She is a girl. I think she is rarely serious and keep joking all the time. She always mock at everything, and never be serious. She always try to make me laugh when I drink (so I could spill it and mess things up) in front of everyone! I really hate her because of this, but I can't show my hatred in front of her. One day I asked her about my objection, and she just said "Serious could make you look older".
What should I do?
What to do to a friend who never serious?
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