Why is the formation of hydrogen bonds between water molecules categorized as cohesion

Because of the polarity of water molecules

Molecular cohesion is the attraction that molecules have for others of their same type. Depending on the intensity of this attraction, a substance is in a solid, liquid or gaseous state.

In this sense, water () can stick to itself through hydrogen bonds, because a water molecule consists of 2 hydrogen atoms attached to 1 oxygen atom.

The oxygen atom tends to monopolize more electrons and keeps them away from hydrogen. Then, it can be said that a water molecule will have a negative side (oxygen) and a positive side (hydrogen).

Thanks to this polarity, water molecules can stick together with the formation of hydrogen bonds to attract a partial positive charge of hydrogen and a more electronegative atom, such as oxygen.