I kinda disagree with this but I’m compelled to put it out there for your consideration. 

Lestat has a huge issue with controlling his laughter but I don’t think he’s depressed or emotionally drained. I think he loves himself too much for that.

I think sometimes it’s more of a nervous tic, a sort of backup reaction to quell confusion, and maybe also as a means of diffusing his own anger. How many lives have been saved by him laughing at them instead of using the Explodey-Heart Gift?

“And my worst problem was laughter. I would go into fits of laughter and I couldn’t stop. Anything could set me off. The sheer madness of my own position might set me off. This can still happen to me fairly easily. … I begin to laugh and I can’t stop…It makes other vampires furious, by the way.” – The Vampire Lestat