however difficult situation he encounters,he always keeps a sense of humor,which(关系代词,代替前文句义) is what（=the thing that） makes him so popular.
1. 状语从句,主句：however difficult situation he encounters,he always keeps a sense of humor.无论处境多么艰难,他总是保持一种幽默感.
2. which引导的非限定性定语从句：which(关系代词,代替前文句义) is what makes him so popular.这一点是.
3. 表语从句：what makes him so popular.什么使他如此受人欢迎.
【翻译】无论处境多么艰难,他都诙谐幽默,这使他非常受人欢迎.