乐此不疲 Enjoy it without tiring
Explanation
乐此不疲是一个褒义词，形容对某事特别爱好而沉浸其中，乐在其中，不觉得厌烦。通常用以赞扬一个人对某事投入的热情和专注。
It describes someone who is very enthusiastic about something and enjoys doing it so much that they don't get tired of it, even after doing it for a long time. It is usually used to praise a person's passion and dedication to something.
Origin Story
传说古代有个名叫王羲之的人，他从小就酷爱书法，每天都练习书法，废寝忘食，乐此不疲。他经常在池塘边练习书法，用毛笔蘸着墨水，在水面上写字。他写得入神，有时甚至忘了时间，直到天色已晚，才发现自己已经站在冰冷的水中，但他仍然乐此不疲。他这种刻苦练习的精神，最终使他成为了一代书法大家。
Legend has it that there was a man named Wang Xizhi in ancient times who had a great passion for calligraphy. He practiced calligraphy every day, forgetting to eat and sleep, and was immersed in his work. He often practiced calligraphy at the edge of a pond, dipping his brush in ink and writing on the surface of the water. He was so focused on writing that he sometimes forgot the time and only realized that he was standing in cold water when it was already dark. But he was still enthusiastic about his work. His tireless diligence eventually made him a master of calligraphy.
Usage
乐此不疲常用来形容一个人对某事非常喜欢，并乐在其中，即使长时间做也不觉得厌烦。可以用于赞扬一个人对某事物的热爱和执着。
It is often used to describe someone who enjoys something very much and is engrossed in it, even after doing it for a long time. It can be used to praise a person's love and dedication to something.
Examples
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他工作起来乐此不疲，废寝忘食。
ta gong zuo qi lai le ci bu pi, fei qin wang shi.
He works tirelessly, forgetting to eat and sleep.
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他每天都乐此不疲地练习钢琴，进步很大。
ta mei tian dou le ci bu pi de lian xi gang qin, jin bu hen da.
He practices the piano every day with great enthusiasm and has made great progress.