How is Gratiano different from Antonio in act 1, scene 1?

In the opening scene of the play, Antonio is in a melancholy mood and is rather despondent and depressed. Antonio's close friends Salerio and Solanio offer several suggestions to explain why he is so depressed. They initially believe that he is worried about his merchandise and ships at sea. However, Antonio dismisses their suggestions and assures them that his investments and ships are not the cause of his melancholy mood. Antonio also dismisses their suggestion that he is lovesick. When Gratiano enters the scene, he also inquires about Antonio's depressed mood. Antonio responds by comparing the world to a play, which happens to be one where his role is a sad part. Gratiano then displays his enthusiastic, carefree personality by saying,

Let me play the fool. With mirth and laughter let old wrinkles come. And let my liver rather heat with wine Than my heart cool with mortifying groans. (Shakespeare, 1.1.82–84)

Gratiano then questions why men act so serious all the time, during his long-winded response, before encouraging Antonio to cheer up and enjoy life. He also urges Antonio to speak more and express his feelings. Gratiano's personality completely contrasts Antonio's quiet, despondent mood. Gratiano is full of energy and mirth. He enjoys his carefree life, does not seem to take anything seriously, and talks excessively.

In the opening scene of The Merchant of Venice, Antonio's feeling quite down, and he doesn't know why. His friends Salerio and Solanio offer their own suggestions as to what the cause of Antonio's depression may be, but all to no avail. Antonio is insistent that he's neither in love nor worried about the loss of his merchant ships at sea. And so he remains trapped in a state of unexplained gloom and despondency.
Gratiano couldn't be more different from Antonio. He's a happy, jolly soul, enthusiastically devoted to wine, women, and song. In trying to get Antonio to snap out of his misery, he cheerfully admits that he's a fool. Gratiano just doesn't see the point of going around with a long face all the time. He says he knows a lot of men who always look serious, and people assume that they're wise. But that's only because they don't say anything; if they opened their mouths, people would think they were fools. So Gratiano is essentially saying to Antonio that it's better to be a cheerful fool (like himself) than a miserable one.