There could be various reasons why this girl laughs at everything you say. Here are some possibilities:
Sense of Humor: She might find your sense of humor amusing and genuinely enjoys your jokes or comments.
Nervousness: Sometimes, people laugh out of nervousness or discomfort. If she's shy or unsure around you, she might laugh as a defense mechanism.
Politeness: Laughing could be a way to show politeness or to avoid any awkward situations when she doesn't know how to respond.
Flirting: Laughing excessively could be a sign of flirting. Some people use laughter to express interest or to show they enjoy someone's company.
Trying to Connect: Laughing can be a way for her to connect with you and show that she enjoys your presence.
Mimicking: If you laugh a lot, she might mirror your behavior and laugh more herself to fit in or to establish rapport.
Attention-seeking: Laughing might be a way for her to gain attention from you or others around.
It's important not to assume the reason for her behavior without more context. Observing other cues in her body language and behavior may provide more insight into her intentions. If you're unsure about her feelings or motives, the best approach would be to have open and respectful communication with her to understand each other better. Always treat her with respect and consideration, regardless of the reason behind her laughter.