Parents - Can't live with them - Can't live without them

I spent last weekend with my folks. It was the first I'd seen them since June. They stopped in on their way home from Ohio to drop off the pool and to help put it up. It was the longest 2 days of my life. Sadly, we didn't get the thing completed so now I'm looking at another 2 days with them to finish it off. Worst part of all - they will be here over my 3 day holiday weekend.
Why do my parents cause me so much grief? Why am I overcome with dread at the very thought of having to spend any time with them? I'm at the point where just contemplating a weekend with them makes me want to skip town. It shouldn't be that way.
Maybe it's the fact that they think they can still boss me around at 25? Maybe it's their constant pessimistic look at everything in life? Maybe it's the fact that nothing I seem to do is ever good enough, even though it's done with all I have? Maybe it's the fact that in their old age they've become incredibly annoying?
Whatever it is, it eats at the very core of me and sets me in the foulest mood ever. I asked them how long they planned to be here this weekend and they couldn't understand why I wouldn't want to work my entire holiday weekend. Hmmm, let me think on that! Mom wanted to know what it was I wanted to do that was more important. As if that's any of her business.
UGH.
She made a comment last weekend about how she figured I'd follow her to Ft. Myers once they moved there. I asked her why she would think that. She said she thought I'd want to be closer to her. I told her 3 hours apart was too close. And it's true. I don't want to rely on them. I don't want to need them. I don't want them to know what I do. They were involved enough when I was a kid. Now I just want my space to be me and live my life. Yet, they can't seem to understand that. I hate being nasty to them, but they don't get the point any other way. And I have to wonder if they even get the point that way.