Disclaimer: Not mine.
It's like a circle, the squad. If someone's feeling something, angry, sad, happy, whatever, it's bound to hit everyone else in the squad by lunch. If someone's got a good feeling about a case, we all kind of pick up on it and generally go with it.
I suppose we go with it because we want to. Especially when it's a good mood. Because that happiness... it's something that we hold on to, every second we can get of it. In this job, you don't find much cause for happiness. Sure, if you catch some rapist or murderer, but then you look back at the family, at the victim, and know that justice can never fully be served and it depresses everyone's mood.
So, we just pass the happiness around, in a circle, hoping it never ends, or breaks.