Let's say you cut a piece of stainless steel in half with a magic knife that maintains the structure of all the atoms but just separates them. That would be an ideal cut; just breaking the chemical bonds that hold the material together. Now, what if you tried to put it back together a few minutes later? Somewhat surprisingly, the pieces wouldn't stick together. The reason is that the surfaces of many materials are different from the bulk material on the inside. In the case of stainless steel, the surface gets oxidized by the atmosphere to make a layer of iron oxide a few atoms thick. This prevents the surfaces from making a perfect match again.
So why does this happen? There are two reasons, really. The first is that oxygen will react with pretty much anything it can get its hands on. It makes especially strong bonds with iron. The second is that when you break a bond, you're actually adding energy to the atoms, and this energy can be used to facilitate a chemical reaction with something else that they come in contact with.
Now you decide to get clever and do this same experiment in space, or a good vacuum chamber. If you still use your magic knife so that there is no grain (crystal lattice, really) mismatch when you put your pieces back together, they should stick.
This phenomenon of separate two pieces of metal stick together does happen in our atmosphere, especially with stainless steel. It's usually not a good idea to use stainless steel screws to hold together something made of stainless steel. If you screw it in really tight, you can scratch off the protective layer of iron oxide on the surfaces, exposing the pure metal underneath, which can then, over time, form new metal-metal bonds. This process is called galling.