Arrangement of elements in the reactivity series
The reason that lithium is on the top of the activity series is due to its behavior in aqueous reactions with water and acid. Due to the lithium atom's very small size (only two energy levels), the lithium ion interacts well with water forming ion-dipole interactions.  The lithium atom's small size compared to sodium and potassium is more entropically favorable in the formation of ion-dipole interactions with water and consequently so are reactions involving lithium ion formation favored to to other alkali metals.  

The reasoning behind this is that reactions favor a greater state of entropy (disorder) in the universe. With larger cations, more water molecules have to become ordered around the cation to form the appropriate ion-dipole interaction.  However, due to the lithium cation's small size, less water molecules are required to form the ion-dipole interaction, thus less order, being more favorable in terms of entropy.