This cropped up in a discussion with a friend - you know who you are.
One of the major reasons why transitioning is such a shitty business is the people who are, in principle, close to you. The logic behind this statement goes like this: when people view themselves as close to a transitioning individual, more often than not they are not close to the individual hirself at all, but to the rôle this individual has played in order to survive without too much harassment. And there lies the rub.
Transitioning requires that that rôle, that act has to be stopped. But, because the close ones are usually heavily invested in that rôle, stopping the act is not necessarily seen as stopping a lie, but is seen as a betrayal instead. The person transitioning, of course, betrays no-one by transitioning, but stops betrayals instead, and this might be the reason why transition is so hard for the near and dear to accept. If they accept the transition for what it is, they also tacitly admit to fooling themselves and their apparently near and dear transitioner, often for decades. They also admit to having policed sex and gender, often in quite backhanded and devious ways - transition brings out all this for everyone to see. It's probably not too easy for the cis people around.
The natural reaction seems to be to blame the transitioner for being manipulative, difficult, intransigent and deceptive. In reality, the transitioner is anything but: a transition is probably the ultimate statement for stopping manipulation and deception.
Another common thread in trying to hold onto the lie is emotional blackmail, and not letting go even when it's perfectly clear that the transitioner wants to have nothing with hir relatives unless they accept the reality of transition. "We do this because we love you" might hold water if transitions were a whim, a passing madness, but as far as I can tell, transitioning is neither - we don't do this stuff just to piss you guys off, y'know.
Eliot put it nicely: "human kind Cannot bear very much reality." (Four Quartets, Burnt Norton, 42-43)
Unfortunately I, and my fellow trans people have to live with this.
[ETA] This trouble with reality seems to undergird a common journalistic trope of trotting out person's trans past, no matter if it's relevant to the discussion or not. It's just so damn impossible for the cis to get that they do make mistakes when gendering other people, it seems.