[@EruditeAssassin] There was no call for a Dex save. Against attacks, you compare their strike value to your armour class, losing ties. Now that you have lower AC than the attack roll, you will take the damage.

On the other hand, when you attack you win ties, so you can think meeting the armour class is the aim of the attacker. Dex saves are rolled when the DM calls for a Dex save (as with all saves, really), for example against the spell Burning Hands the saving throw is made. Another example could be falling rocks. I hope I have been of assistance.