Dronacharya demanded from his Ekalavya after he saw that by his arrows alone, Ekalayva was able to stop the dog from barking. Some people say that he secretly wanted Arjuna to be the best archer and asked that demand but my question is, was Arjuna happy or sad? What was Arjuna's reaction?
Arjuna was happy as per Mahabharata: Adi Parva: Sambhava Parva:
"Vaisampayana continued, 'Hearing these cruel words of Drona, who had asked of him his thumb as tuition-fee, Ekalavya, ever devoted to truth and desirous also of keeping his promise, with a cheerful face and an unafflicted heart cut off without ado his thumb, and gave it unto Drona. After this, when the Nishada prince began once more to shoot with the help of his remaining fingers, he found, O king, that he had lost his former lightness of hand. And at this Arjuna became happy, the fever (of jealousy) having left him.