When someone inherits a Titan Shifter in Attack on Titan, they gain access to the previous holder's memories. This creates memory chains, but because memories can be diluted or even difficult to visualize, it makes sense that just because a character can see their predecessor's memories doesn't mean they can vividly remember everything. Another effect of memories can be how one feels and perceives the world around them. Memories are linked to emotional impressions of people, places, and events, and it is said that people do not remember what someone said or did as much as how that action or event made them feel.
Armin Arlert was the Colossal Titan's heir after he was transformed into a Titan and devoured Bertholdt Hoover. Bertholdt's feelings for his fellow warrior Annie Leonhart were clear throughout the story. Armin even manipulated Bertholdt so Erwin Smith could catch him off guard. However, when Armin gained the Colossal Titan's powers, he also gained Bertholdt's memories, and thus the feelings associated with those memories. The question is whether Armin and Annie's future relationship was foreshadowed before he became the Colossal Titan, or whether his feelings were fueled by inheriting Bertholdt's memories.
Were Armin's Romantic Emotions for Annie Predicted?
There is little to go on in terms of romantic affection based on the two characters' initial interactions. Armin appeared to admire Annie before concluding she was the Female Titan, and upon the reveal, he demonstrated that he desired a deeper understanding of her and the warriors' motivations. Armin is a character who uses his intellect to naturally unravel people's motivations and feelings, allowing him to predict their next actions or what will cause them to make certain decisions. Armin's use of this method on Annie compelled him to seek a more in-depth understanding of who she truly was.
Armin spent a lot of time visiting Annie in her crystallized state after receiving the Colossal Titan, informing her about what was going on in the world. This interest in her could have stemmed from both his own curiosity about who she was and Bertholdt's memories influencing him. It does not appear that when someone inherits the Titan, they immediately feel the same way for someone as the previous holder; however, a combination of Armin's desire to understand Annie and now seeing who she once was through memories sparked a more romantic interest — more so than simple admiration or curiosity.
Annie's Love for Armin Returns
There is some debate about Annie returning Armin's feelings because it is also regarded as a haphazardly imposed relationship. Annie had the opportunity to kill Armin during the first reveal of the Female Titan and her attack on the Scout Regiment's decoy formation. She didn't because she was looking for Eren Yeager, but Reiner Braun interrupted her before she could. The question is whether she would have killed Armin there or Jean Kirstein when he was vulnerable. Would she have murdered any of the 104th trainees? Annie demonstrated a willingness to kill for the sake of her goal, but Armin, a major threat to her advancement, was spared.
Armin was most likely drawn to Annie because of her history and actions, and his inability to fully comprehend her motivations kept him interested. Armin grew a greater sympathy for Annie's situation with the inclusion of understanding provided by Bertholdt's memories and the emotions associated with those memories, and the continuous interactions he had with her built up his romantic feelings over time. In short, while their romantic involvement was not foreshadowed, it was also not completely unexpected. In reality, romance is not always predictable and can certainly develop over time rather than being explosive from the start – despite the fact that many anime tend to take the opposite approach.