Algodoo

Actually, this is not weird at all. For the boxes with + attraction, all four fixed boxes are pulling on the center box with equal intensity. But if the center box just happens to move slightly toward any of the fixed boxes, then it will stick to the box like a magnet. For the boxes with - attraction, all four fixed boxes are pushing on the center box with equal intensity, and so the exact center area between the fixed boxes is where all four forces are equal, and so the small box tends to stay in that central area where all forces around it are equal. For the boxes with mixed + and - attraction, the small box will move toward and stick to one of the two boxes with + attraction. If you give the box a little push, it will move toward the upper or lower box and will stick to it.

So, the bottom line is, the small box behaves exactly as expected. Nothing weird going on at all that I can see!

i just wanted to add somemistery

Sorry!

Cool

The + attraction is like an unstable pendulum. The small push/pull exerted from the other boxes on the left causes it to stick to the right box, while the attraction - is like a stable one; it will always try to go to the center of the box, and stop there eventually if there is some sort of damping (just like a real pendulum!) like air friction.

The +- box is... I don't know what analogy to make for it. It only sticks to one of two boxes, rather than one of four.