What do you think about me stealing someone's spot at lunch?
Why are you going to do that?
I just want to sit down for a while, and this is a good spot
You really wanna cause a possible conflict over a seat at lunch?
The seat was there and I'm taking it, and if the person doesn't like it, they can lump it.
Since it's in public, you have the right to sit wherever, but don't be surprised if they get mad at you.