I see. That makes plenty of sense. Tidus's automatic response to any man associated with the Church is to doubt their credibility. It's no fault of his own, given his past experiences with Yevon were mainly negative. (Hey! Could you say Tidus based his first impression of Bria a posteriori given his resentment towards Yevonites?) So naturally wherever Yuna is concerned, he's highly protective of her.
For an extroverted teenager, Tidus is surprisingly introspective. Normally I would have called that out-of-character, if it weren't for his constant internal monologues in FFX. Not that I ever found them annoying or lame, I honestly enjoyed his narrative voice. It's just easy to forget he's capable of self-reflection (which many others tend to forget, too). Unlike Square Enix's poster boys of positivity, Zack Fair, Zidane, and Sora, Tidus has more than his fair share of insecurities.
Okay, how about this: Would you say Seymour wanted a causal relationship with Yuna to become the Final Aeon? Twisted, romantic feelings notwithstanding, all of his efforts forced her to do whatever that benefitted him in his end goal. As for Yuna's feelings for Tidus, would you say she developed them a posteriori? She fell for him, because Tidus embodied everything she craved: freedom, courage, and change. Because of grief and loss, first her parents, then one of her childhood friends, tradition and expectations stifled her, but she chose to follow them because they were familiar. But at the core of every Summoner, they wish to usher in change - the chance to fell Sin permanently.
I learned a valuable lesson in my Acting class about Characters and their tactics: No matter if it's selfless or selfish, we always form relationships with people when we want something. Intentionally or not, when we become friends with someone, or wish to pursue a romantic relationship, we want something out of it - let it be safety, money, security, affection, validation, etc. The feelings we develop for someone or something can be considered a posteriori, or even a priori because we want to experience what we never experienced. Heck, even Freud's theory that we are attracted to people who remind us of our parents of the opposite gender (for better or for worse) can be one type or manifestation of a posteriori.
Or am I getting too ahead of myself? XD