obsession       
noun [ C or U ] uk/əbˈseʃ.ən/ us/əbˈseʃ.ən/
B2 困扰;无法摆脱的念头;念念不忘的事(或人)
something or someone that you think about all the time
  an unhealthy obsession with death 总是想到死的不健康念头 
 her chocolate obsession 她对巧克力的痴迷 
 He's always wanted to find his birth mother but recently it's become an obsession. 他一直想找到自己的生母,但最近这成了他的一块心病。 
 - More examples
 - He has an obsession with cleanliness.
 - His campaign for fathers' rights has become an obsession.
 - The kids have an obsession with computer games.
 - I've got a bit of a jewellery obsession.