"Why does Ti always go with Fe and Fi with Te?"
actually this relationship exists mostly in J-doms.
and the opposite J is their shadow.
so you can say that it "comes along with" the opposite J and you can say that it "goes against" the opposite J.
and "why" is a complex question. because god wanted it to be this way? because it has been observed and was written down?
but it's not surprising that the shadow works against "you", is it?
and i don't think it makes sense to define a shadow as something entirely static.
it responds to you. if you are P-dom and use your secondary J a lot, your shadow might just respond with your opposite J a lot.
the shadow has all the possibilities, but it has probabilities as well.
in the big picture of your live, your decisions are created by your dominant function and they are kept in between boundaries by the opposite of your dominant function.
and why is it so easy to observe the J-J conflict?
because J functions can be more easily identified via theory of mind, if communication is the object ob observation. they corrupt interpretations and leave an obvious handwriting. the P functions can be identified by actions, readiness and certain skills - factors that are comparably elusive in communication.
is there a necessary conflict?
not in theoretical utopia.
don't confuse functions with their contents or conditioning:
the developmental learning task of every function is to work together not against other functions.
example: if you are Fe-dom or Fe-secondary:
to develop your shadow-Fi you must access Fe consciously, understand what it does and why, then stop Fe from discriminating Fi. then Fi will start to talk to you and say what it wants.
a matter of adjusting your understanding and attitudes and forgiving and giving up specific attachments to ideas or preferences.
this is true for the opposites of introversion in the same functions. (Ti vs Te, Se vs Si, ....)
----
i don't see a hard opposition between introversion of one function and extroversion of the other function. (Ti vs Fe, Te vs Fi, Se vs Ni, Ne vs Si)
this is, i believe, why those shadow qualities become so obvious.
a Ti-dom is unable to suppress his Fe because he does not even recognize it. so the Fe shadow becomes a mighty live force. but it has very odd characteristics, compared to the Fe of an enfj, because it's not flexibly concerned with the immediate situations but with some general live theme, like a code of honor of a soldier for example. so Ti-doms are enslaved by Fe and any conscious conflict with Fe is a result of this, not the origin, but more often there is no conflict and Ti-dom are stubbornly identified with their Fe attitudes, similarly to how someone "is identified with an offender" in a stockholm syndrom.
likewise the Ni-dom is unable to suppress Se, so Se becomes a mighty active ruler of the Ni-dom's live theme. as such it's enslaving not empowering. its the voice of paranoia. dooms-day dreams.
so this kind of conflict is hardened by conditioning at it needs some kind of therapy in the broadest sense of the world to ease the conflict.
---
but Ni-dominance can suppress Ne, so the lack of Ne becomes a dominant factor in live.
and Ti can suppress Te so the lack of Te can become a dominant factor in live.
so this is a hard and direct conflict (Ni vs Ne, Ti vs Te, Se vs Si, ...) which is often conscious.
but i believe such conflict can naturally be transcended by sort of loosing yourself in some kind of demanding activity.