Not in love neither? Then let us say you are sadBecause you are not merry—and ’twere as easyFor you to laugh and leap and say you are merryBecause you are not sad. Now, by two-headed Janus,Nature hath framed strange fellows in her time.
Solanio tells Antonio that they'll just consider him sad because he isn't happy.... but it would be just as easy to act as if he were happy and in a good mood.