The character of Lupin is a very interesting one. The treatment and discrimination he faces from the wider wizarding world has (quite understandably) corroded his spirit. He allows himself to be overcome by a certain amount of self-loathing and self-pity. And I make absolutely no judgement about that, I actually think this makes Lupin one of the most interesting characters. I'm not saying whether it's right or wrong, or acceptable or unacceptable, or whether you and I could do any better, but it leads him to a number of self-destructive behaviours.
The most notable of these is the way he behaves towards his wife and child.
He does not initially want to "inflict himself" (my words) upon Nymphadora Tonks:
'And I've told you a million times,' said Lupin, refusing to meet her eyes, staring at the floor, 'that I am too old for you, too poor ... too dangerous ...'
'I've said all along you're taking a ridiculous line on this, Remus,' said Mrs Weasley over Fleur's shoulder as she patted her on the back.
'I am not being ridiculous,' said Lupin steadily. 'Tonks deserves somebody young and whole.'
Harry Potter and the Half-Blood Prince - p.582 - Bloomsbury - Chapter 29, The Phoenix Lament
'Don't you understand what I've done to my wife and my unborn child? I should never have married her, I've made her an outcast!'
Lupin kicked aside the chair he had overturned.
'You have only ever seen me amongst the Order, or under Dumbledore's protection at Hogwarts! You don't know how most of the wizarding world sees creatures like me! When they know of my affliction, they can barely talk to me! Don't you see what I've done? Even her own family is disgusted by our marriage, what parents want their only daughter to marry a werewolf? And the child - the child -'
Lupin actually seized handfuls of his own hair; he looked quite deranged.
'My kind don't usually breed! It will be like me, I am convinced of it - how can I forgive myself, when I knowingly risked passing on my own condition to an innocent child? And if, by some miracle, it is not like me, then it will be better off, a hundred times so, without a father of whom it must always be ashamed!'
Harry Potter and the Deathly Hallows - pp.175-6 - Bloomsbury - Chapter 11, The Bribe
In this extract we see, further, how he feels about himself, his condition and his decision to have a child.
And so he walks out on them.
Even when he is with Tonks, he is not open with her, he hides himself from her, and envelops himself in remorse for having married her and in misery at the discrimination he suffers:
Although Lupin smiled as he shook Harry's hand, Harry thought he looked rather unhappy. It was all very odd; Tonks, beside him, looked simply radiant.
Harry Potter and the Deathly Hallows - p.101 - Bloomsbury - Chapter 7, The Will of Albus Dumbledore
'Sorry about last night,' she added in a whisper, as Harry led them up the aisle. 'The Ministry's being very anti-werewolf at the moment and we thought our presence might not do you any favours.'
'It's fine, I understand,' said Harry, speaking more to Lupin than Tonks. Lupin gave him a swift smile, but as they turned away, Harry saw Lupin's face fall again into lines of misery. He did not understand it, but there was no time to dwell on the matter: Hagrid was causing a certain amount of disruption.
Harry Potter and the Deathly Hallows - p.116 - Bloomsbury - Chapter 8, The Wedding
How much you blame him is up to you, I find the character of Lupin really really interesting. And this all speaks to one of the darker, more insidious results of discrimination and what it can give those who suffer it to do, how it can destroy their lives - by, sadly, negatively affecting their actions. This can have a horrible effect on those close to them, as we see here, because suffering discrimination can cause people to act very badly.
Nevertheless, I think the example of Remus Lupin is an exquisite warning to us all about just what will help us and what will harm us. And, again, a bad action is a bad action, however malignant your circumstances. It was Remus Lupin who directed his own legs out of the door.
But the point here is that Dumbledore knows this about Remus Lupin. He is much too wise to try to persuade Lupin out of this mindset.
'Goodbye, then, Remus,' said Dumbledore soberly. Lupin shifted the Grindylow tank slightly so that he and Dumbledore could shake hands. Then, with a final nod to Harry, and a swift smile, Lupin left the office.
Harry Potter and the Prisoner of Azkaban - p.310 - Bloomsbury - Chapter 22, Owl Post Again
He is resigned.
It is true, I suppose, that Dumbledore could bewitch, or hoodwink Lupin into staying, but he cannot exorcise Lupin's demons. He can make Lupin stay, he cannot make Lupin want to stay.