What makes DNA sticky-ends sticky?
What are sticky ends.
There is no substance that is attached making the DNA ends "sticky". What has actually happened is an overhang of at least a few nucleotides. Blunt ends are another kind of cut, but have no overhanging residues.



Why are sticky ends sticky?
Restriction enzymes usually cut these ends deliberately so that a four nucleotides are overhanging on the 5' end of the double strand. These are complementary to other overhangs and because they are less stable than a bound double strand region, and are able to hydrogen bond easily with complementary bases, they are easier to attach with a ligase.

Below is an image of the base pairs interacting via H bonds.