will-o'-the-wisp
/ˌwɪl ə ðə ˈwɪsp; ˌwɪləðə`wɪsp/ n
1 bluish moving light that may be seen at night on marshy ground 磷火; 鬼火.
2 person or thing that is impossible to catch or reach 捉不到或接触不到的人或事物
You shouldn't hope to find perfect happiness it's just a will-o'-the-wisp. 不要指望能找到十全十美的幸福--那只不过是镜花水月.