Is it normal that i laugh hardest when i'm sad?

Normal

Yeah, sometimes when I'm very sad or stressed I'll stumble across something funny and laugh as if it's the most hilarious thing I have ever seen in my life. As Artbuggy said, it's just the body's way of releasing some of the negative feelings.

Yes. It's a natural human instinct called "nervous laughter." :)

I think this happens when you don't have a good outlet for stress or sadness in your life. and so it ends up in your laugh.
It happens to me too, because I dislike the idea of dumping little problems of mine onto my friends and family I end up keep most of it inside.
I've been described to have a big personality but sometimes I'm using that big personality to hide big pain.
If something has been on my mind or bothering me all day, then someone says something surprisingly funny I just crack, at worst I laugh hysterically and cry a little then hide it fast, and at best I laugh harder than the joke deserved and get a weird look.
It's just the body's pent up stress coming out, normal.