Why are hexagons so common in nature?

It's one of the only shapes which tessellates perfectly (think tiles, if you tiled a wall with hexagons then there wouldn't be any gaps. Of the shapes which tessellate perfectly (triangle, square, hexagon) it has the smallest perimeter for its area i.e. a square of area 10cm^2 has a greater perimeter than that of a hexagon with an area of 10cm^2.

So, if you were a bee trying to make a honeycomb then the hexagon is best because it would use least wax to make a certain area of comb (because of the perimeter/area ratio) and also not waste any space (because it tessellates without leaving any gaps).

Hope that helps.