Here are my reasons and a piece of evidence for those reasons.
Reason 1: she’s extremely selfish
Evidence: episode Frightningale
Marinette and her friends try out for the back ups to a music video made by famous songwriter Nightingale. Adrien just so happens to be playing the part of Chat Noir and none of the Ladybug tryouts work out so Nightingale puts up a test ti see who would do what Ladybug would do then pick them. Marinette happens to be her choice but Mari declines so she stays with her friends and keeps her identity hidden, while getting ready Chloe walks in and takes the part of Ladybug. After seeing this Mari stands up and quickly says she changed her mind and wants to be Ladybug simply because she hates Chloe and she hates the thought of her performing with Adrien, and this decision ultimately leads to the almost reveal of her identity and the akumatization of Nightingale. She put her identity in danger and angered the daughter of the mayor for no other reason other than “Chloe and Adrien cannot preform together.” Like gurl what. Let Adrien decide for himself who he likes instead of taking every opportunity from other girls just cause you like him.