A proposition is a causal claim if it takes the form that allows one to predict y from x, where x can be anything. A reason is a account provided to make a set of behaviors intelligible. Some propositions can be understood as reasons or causes. Notice that with regard to very odd, bizarre, seemingly irrational behavior, reasons just seem uncalled for. One wants to know the causation of serial killing, e.g., perhaps, abusive child-rearing practices.
Intentions are more like reasons than causes, and are usually couched in goal-directed language.
So just having an intention is no guarantee of reaching the goal. Now, the question you raise is whether we should attribute an intention to a person who does not have the wherewithall to accomplish the goal or is clearly acting at variance with the stated goal. In this case, I can accept that the lover never intended to reject his loved one, but, unfortunately, was an incompetent lover. I'll it to you to worry this problematic claim. Of course,it may well turn out that this young person never gets aids.