1. eager to take on difficult or challenging tasks
he/she is a glutton for punishment
A 'glutton' is a greedy or excessive eater. A 'glutton for punishment' is someone who is greedy for unpleasant or difficult tasks. Someone who is a 'glutton for punishment' may choose the more difficult option than an easier one in order to get a better result. You will hear this used in social and professional contexts.