I am 25 and the generation is full of attraction towards the opposite gender, most of the time there is heartbreak. So does rejection come under Adharma?

No, in youth you will behave like Shankaracharya said in Bhaja Govindam – when you are a child you will be obsessed with toys, when you are a youth, you will be obsessed with the opposite gender, and an old person will be attached to worry and anxiety. This is a part and parcel of life. But in and through this, if you follow the teachings of the Gita, you will be able to achieve outstanding success. You will not face rejection and you will be a success in life. So, do what you have to do but invest a little bit of your time and effort in studying the Bhagavad Gita. In enriching yourself from within.