Dear K.,
Ahhhhhh, laughter....the old diversionary tactic.
While it's true some kids may laugh while you are trying to discipline because they don't take you seriously, often they laugh because they are hoping YOU will laugh and they are thinking, "Hmmm, I just did something I shouldn't have. Think fast.....okay.....I'll try pretending it's cute and funny and maybe Mom won't notice." It's a way of trying to change the subject at hand.
A lot of kids try it.
I believe in laughter. I have always joked around and played and been silly with my kids, and my kids have great senses of humor, and because of this, I had to be sure they understood there are things that are funny, there are times to be funny, and there are things and times that just aren't.
Hitting, biting, tearing up books....that's just not funny.
I would say, "I think it's really funny you are going now to stand in the corner for 10 minutes and laugh at how funny the corner is. 5 minutes because you are 5 years old and another 5 minutes so you have time to pray you will learn to love this corner. We'll see if you're still laughing after 10 minutes because then you are going to clean the mess you made, apologize for biting (or whatever), then you are going to go back in the "laughing corner" for 5 more minutes. Nose to the corner, I want to hear you laugh about how funny this is. I'm timing you."
If you do something like that one time over the serious stuff, the little things will fall into place.
There's funny, and then there's mom being dead serious.
Telling your daughter to stop laughing isn't working and she already said she doesn't know why she does it. Stand her in a corner, just her and a 90 degree angle and the fun will wear off. You just have to let her know that cute and funny won't get her out of everything.
Standing in the corner for a while actually gives a kid some time to think about Plan B. Such as, "I will get in trouble for this....maybe I shouldn't do it next time".
The Plan B concept takes some practice and contemplation. And, a little reverse psychology once in a while.
Best wishes!