since the beginning, annie was pitying armin or treated him special as she had a chance to do it before but she didn't armin felt a certain feeling towards annie but after knowing she is a traitor he kept trying to forget about her until she was in the crystal. he visited her every day because he truly cared for her and he missed seeing her and thought that she might have her own reasons to be like this. She felt him through the years as he came and thought that it was so nice of him. after she was out of the crystal and she knew that he visited her everyday even tho she was a traitor, she felt love towards him as well and was standing beside him in the fight against erin . until erin was dead and the Titans were gone.. they had no more worries so they both tried to spend more time together and build more feelings toward each others