pull a long face字面意思拉长着脸，说明某人心情肯定很低落，有点不开心，所以就是指“愁眉苦脸、不高兴”。

【例句】
It's no use to pull a long face; no one can help the fact that it is raining.
愁眉苦脸是没有用的，天要下雨时任何人也改变不了。

Why do you pull a long face? What has happened with you?
你为什么闷闷不乐？发生什么事了？