Hi Safaa, this is from "As you like it", Act 1, Scene 2. Orlando comments, "I, how bitter a thing it is to look into happiness through another man's eyes" . He is complaining that in spite of his brother's happiness, he will be depressed the next day during the wedding because he wants to be with Rosalind.

Hello Safaa, He wanted this happiness in particular to be his and this girl to belong to him, while it was going to be his brother's instead. He finds it thus a difficult and bitter situation to have to endure such kind of pain.

Hi... It means happiness of others is sometimes very painful for us...