Why do dogs lay on their backs?

Now THAT'S a question that has been bothering me for a bit over a year. I always thought that it was because they wanted to cool their belly off... I was so, so, wrong.

I did a little research today, and found out that if a dog is lying on his or her back, it could mean three things. First, it means that he or she is accepting you as his or her master. No WONDER my dogs always lie on their backs..
It could also mean that they are completely relaxed, and it just feels... good. It's the same feeling that us humans have on the beach, when we're just lying down under the sun, listening to the waves, letting the wind blow over our bodies... Damn.. Who wants to go to the beach tomorrow?
After hearing all that, you're probably thinking "so if a dog lies on his back, it is most definitely a good thing,  right? Am I right, Mr. Officer?". You wish. You're never right.

I'm kidding :D

Anyways, if a dog lies on his back with his paws covering his chest, then it means "don't touch me, don't pet me, don't do anything to me". You know, now that I think about it, that might be the reason my dogs hate me!
No, actually they don't, but they do sometimes ignore me.