I am not an INTJ, but I know INTJs inside and OUT!
and as an INFJ male I kinda think I know where your coming from with this question.
First and foremost is to understand the core of the type, and the core of the INTJ is this...
"PROVE IT"
"Effective"
that's it, everything will go back to this no matter what, these two factors are stronger than what you as an INFJ will consider to be "love"
to answer your question why would an INTJ male end a relationship or get out of a once "effective" relationship is simple. The relationship is simply not "effective" anymore. This is not about having fun....enjoying company....INTJs are not "fun" people they are more sick in the mud types. INTJs are about controlling the situation to "feel" as they are feeling(Fi). IF they are happy and getting drunk GREAT everyone party up....if they are being reserved and prude...than you better stop partying! they dont give half a shit how the group is feeling. take that one further they dont give half a shit how you are feeling.
now they are not demons....they dont wanna hurt you per say or want you unhappy...but hey you need to get with THIER program....or this relationship is simply not effective.
Now an INTJ will with ALL their glorious logic will state the this effective or lack of effective relationship is best if ended for both parties. they will say something like they have looked at for Your side as well, and this is a Half truth....yeah they most likely did and thought well its better for the INFJ to be with someone that loves them they way they want and need....BUT DONT for one moment ever think that they thought of that FIRST, no way in hell. No they thought of how the relationship is not EFFECTIVE for them! and than rationalized it for you with out your consent.
NTs are about "rational love" love because it makes sense because its effective, NOT what you would think of as love being a powerful... higher power that is beyond words AND emotions, its love INFJ love simply is, and there is no sacrifice too great for love, and love means something.
see the difference?
remember INTJs are Fi users....and that says ALOT. I honestly cant deal with Fi users well, unless they are NFPs for some reason they are not as bad.
I dont think an INTJ will grow tired in a relationship, there are not xNTPs that get bored, so that would not be the answer its the idea that the relationship is not effective for them....and effective could be alot of things but the effective is the root of this topic,
also remember that something that is effective.....doesnt take alot of HARD WORK. something that takes hard work is usually not effective, so ponder that.
If any INTJs take offense sorry YO!.... but I am not really sorry! HAHA!
Also dont argue with an INTJ, its pointless, they wont give in and in a relationship they know they are wrong, cause they are selfish and a relationship is about 2 people..... but it wont matter and in the end you will get upset and angry and they wont care they you are angry anyway. this doesnt mean they win, in fact in this case and INTJ losing an INFJ is worse for the INTJ....the INFJ will gain freedom, emotional strength sanity. and hopefully an emotional secure and intelligent partner.
The INTJ loses more than they could ever understand and by the time they figure it out its too late.