Irony and Pity

In chapter twelve, Bill tells Jake to "show irony and pity" (page 118). He starts randomly singing "The Bells are Ringing for Me and my Gal." I had no idea what this song was, so I decided to look it up. Here are the lyrics:

What a beautiful day
For a wedding in May!
See the people all stare
At the loveable pair.

She's a vision of joy,
He's the luckiest boy.
In his wedding array
Hear him smilingly say:

The bells are ringing for me and my gal,
The birds are singing for me and my gal.
Everybody's been knowing
To a wedding they're going
And for weeks they've been sewing,
Every Susie and Sal.
They're congregating for me and my gal,
The Parson's waiting for me and my gal.
And sometime
I'm goin' to build a little home for two,
For three or four or more,
In Love-land for me and my gal.

See the relatives there
Looking over the pair!
They can tell at a glance
It's a loving romance.

It's a wonderful sight
As the families unite.
Gee! It makes the boy proud
As he says to the crowd:

The bells are ringing for me and my gal,
The birds are singing for me and my gal.
Everybody's been knowing
To a wedding they're going
And for weeks they've been sewing,
Every Susie and Sal.
They're congregating for me and my gal,
The Parson's waiting for me and my gal.
And sometime
I'm goin' to build a little home for two,
For three or four or more,
In Love-land for me and my gal.

I still do not know why Bill started singing this? When Jake first woke him up to go fishing, Bill was in a bad mood and arguing. Now it is only a few minutes later, and he is randomly singing this song...which seems like a happy song. Why did Bill randomly change from arguing to being happy? And what was the significance of singing this song? I am sure there must be some kind of reasoning for this spontaneous song...but I certainly cannot think of a reason.

...any ideas?