Why must i always contradict my best friend?

We used to be best friends but now everytime she says something i find it a habit to contradict her. I'm trying to stop, but I think it is because I can manipulate her decisions greatly and also because i feel like i have some power. Any other reasons maybe?
Answer:
you merely see flaws in her thinking because you are both wired differently. I think it could be a good balance to have between the two of you. both sides of a situation are always weighed.

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you feel inferior to your friend and this is an attempt for you to show her and yourself who's the boss. your own insecurities are messing up your friendships

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