How does Solanio comfort Antonio in act one scene one?

Solanio attempts to find the cause of Antonio's sadness. He cannot find an actual cause of Antonio's melancholy so he suggests Antonio is just in a bad mood. He tells Antonio to act happy because it is just as easy as acting sad:

Then let us say you are sad

Because you are not merry—and ’twere as easy
For you to laugh and leap and say you are merry
Because you are not sad. Now, by two-headed Janus,
Nature hath framed strange fellows in her time.