When they first met, Astrid, Hiccup, Toothless and Stormfly where under attack by Eret and his dragon trappers, Eret believing that they were accomplices to the destruction of their fort by Valka and her Bewilderbeast. After escaping, Hiccup and Astrid returned (somewhat) willingly in order to get to Drago Bludvist and convince him to stop his war against the world. From then on, Eret were placed under the mercy of the Dragon Academy, threatened and tormented by Astrid in order to locate Drago, thinking that Hiccup was with him.
When Astrid was not interrogating Eret, she would often leave him to the mercy of her dragon Stormfly, who would play with him like a cat with a mouse-toy. When Drago tells his men to kill Eret, feeling that he has outlived his usefulness, Stormfly comes in a saves him. This leads Eret to the realization of everything Hiccup having said about dragons to be true, taking out the guards that intended to throw them off the ship into the freezing ocean and freeing the rider's dragons. When freeing Stormfly, he formally thanked the nadder for saving his life, becoming her second rider during the seige of the Dragon's Hide.
While not overtly romantic, Astrid and Eret's relationship evolve into a genuine friendship, allowing elbow room for something more to blossom, as opposed to the one-sided "played-for-laughs" Ruffet.
Note: Please add only ones that are part of the RotBTD fandom