In this case, atmospheric pressure does not comes into it because it acts in all direction. The force which is effective in case of water between two pieces of glass is adhesive force. As, adhesive forces are considered that between two different bodies; cohesive forces are internal forces of a body, resulting from attraction between the molecules of it. The attractive force between water and glass (the glass contain silicium atoms, negatively charged and water is a polar molecule so that the positive side of water is attached and causes part of the bound) keep them firmly tovether. Due to the big surface of the glass slide, the resultant force is also big. So we have to apply a large force in order to separate two glass plates enclosed with water film.