Because she is the only one who told that she will kill Titans. Had anyone told that they will kill Titans?
SJC-Having a good life
Hange-crazy about Titan
Erwin-to prove his father correct
Levi- nothing or following Erwin
Overall, no one give shit about Titan and Eren is crazy about killing Titan, when someone shares your dream you automatically start liking them.
Also, Historia impressed him throughout the Arc, when Eren was going low she was going high.
In the farm scene, you can see how serious he was throughout the conversation.
He bypassed the Jean's joke
Told that he will kill RB
So, an the Snk girls he hold her in very high regard which you can call it as respect. Gradually feeling might have increased during the years.
While Historia started liking him after Ch 107.