In The Merchant of Venice, why does Gratiano say, "Let me play the fool" in Act 1, scene 1?

Gratiano has just told Antonio that he does not look well and that he is much too serious. He tells him that those who care too much about the world tend to lose the little they have. He says that Antonio does not look his old self.  

In reply, Antonio tells him that his vision of the world is just that - it is the world and nothing more. It is a stage on which every man plays a part and his one is a sad role. 

Gratiano suggests that if the world is as Antonio says, then he will play the part of a jester who will encourage laughter, causing faces to wrinkle. He would rather seek pleasure in wine and harm his liver than burden his heart with sad groans which could kill him.

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...

Gratiano then asks why a warm blooded man should sit like an old man as if he is made of stone and be asleep when he is actually awake. He further asks why any man should become ill by constantly being irritable. It is obvious that Gratiano wants Antonio to adopt a more pleasant demeanour for he then tells him that he loves him and that he is advising him out of love. 

He mentions the fact that there are many men who hardly ever speak. They develop a certain air about them by being silent. These men adopt a stern look and their faces look like stagnant ponds covered in filth. They attempt to create the impression that they are wise and philosophical men who deserve respect. Their superciliousness demands that others should be quiet and listen when they speak. Even dogs should stop barking.

He furthermore states that he knows such men and that they are only deemed wise because they hardly ever say anything, but when they do speak, it becomes evident what fools they actually are. He concludes his speech by saying that he would lecture Antonio some more about the subject at a later occasion. He suggests that Antonio should not use his melancholy as bait for him to offer any further opinions, for he is but a fool.

But fish not, with this melancholy bait,
For this fool gudgeon, this opinion.

He then asks Lorenzo to leave with him and greets, stating that he will continue trying to encourage Antonio after dinner.

Gratiano's attempt to encourage Antonio out of his melancholy is quite strange, to say the least. In order to cheer up his friend, he rather criticises the negative qualities of a sombre mood instead of saying something positive, although his suggestion at the start is that he would play the jester and encourage mirth. His speech comes across as more of a stern lecture than an inspiration. I am sure that if I were Antonio, I would be even more depressed after such a 'cheerful' talk! 

As it is, Antonio's mood is not lightened at all and he, somewhat sarcastically, remarks that he will now speak much more after Gratiano's lecture.