I disagree with everything you said. First and foremost, a successful relationship is not based on how much alike you are. It is based on how well you fit each other's needs. Oliver and Felicity did that for each other. When Oliver stopped killing, Sara continued. Oliver was changing, evolving and Felicity was a major reason why. Also, the connection can be seen as early as their first scene together, which is why Stephen Amell pushed for Emily Rickards to stay. You must be watching a different show if you think Oliver fell in love with her in the middle of this season. You should rewatch it. Finally, it's CW, romance is their thing.