why did ekalavya cutted his thumb

Long ago there lived the Pandavas and the Kauravas, the princes of Hastinapura. All the princes were taught archery by Sage Dronacharya. The Pandava prince, Arjuna was Dronas favourite student. Ekalavya, a poor commoner boy also wanted to be Dronas pupil but Drona refused to teach him.

Ekalavya was a determined boy. He carved a statue of Drona on a tree trunk in the forest and started practising in front of it. Time went by. One day the princes and their teacher came to the same forest. As Arjuna aimed at a particularly difficult target, an arrow pierced the target. Shocked, the boys and their teacher looked around. They saw Ekalavya, who went up to touch Dronas feet. Who is your teacher? Drona asked. Ekalavya quietly led him to the statue. Drona did not want anyone to be better than Arjun. He thought for some time and said, If I am your guru, give me my gurudakshina. Surely, sir, bowed Ekalavya. I want your right thumb, replied Drona. Ekalavya wordlessly cut off his right thumb and laid it at Dronas feet.