How did Sara Ali Khan and Kartik Aaryan go from being best friends to mocking each other in public?

Sara and Kartik looked like they were madly in love when they were promoting Love Aaj Kal. A wave of bad luck or baby evil eye came in their way of relationship which made things worst. It is despicable when you see your ex commenting things like Sara Did About Kartik. It was definitely not expected from her.