Why does Sydney Carton despise himself so much?

I have read suggested somewhere that Sidney Carton had syphilis. Dickens could not be explicit about sex matters. I suspect often when people hate themselves it has something to do with their sexual proclivities. Sydney Carton is heterosexual, so maybe his problem is that he cannot keep himself away from the prostitutes. If that was the case he may have contracted French disease. Or he may just despise himself for seeing prostitutes, particularly if he has had a religious upbringing. Sydney Carton is clever (he's another lawyer, isn't he), he has enough money, and if he looks like Charles Darnay then he's handsome too. Why doesn't he think he can get himself a nice, young lady like Lucie Manette?

I energetically refute the suspicion of Sydney Carton having syphilis without a convincing proof!
He is a drunkard.I think he simply despised himself for though being clever, learned and good looking(I think he didn´t have much money), he isn´t sucessfull. In this point he is the opposite of his fellow lawyer, who exploits his intelligence and his creativity.S.C is one of the creatures that having all the means to be sucessfully personally and professionally remain outsiders to the system.
One knows people in the real world who are like that. It may be a critic on the ruththlessnes of capitalism. And he had to be like that to be able to redeem himself later.

He is not successful at work, but he could be. I think he does not try because he sees no point. He cannot be happy because he has the pox. You can't marry a sweet, pure, virginal, young lady, similar to Lucie Manette, and give her the pox. Your babies would be miscarried, stillborn or handicapped, and you could never be sure whether the disease will come back and kill either yourself or her. There is no point in the big house and carriage. I think that's why he's so gloomy, and drinks so much.

Maybe he just needed to wait a couple of years. After the primary and secondary stages, syphilis enters a latent stage. Syphilis is still a bit infectious in the early latent stage, but after a couple of years is much less so. Then he may well be lucky because tertiary syphilis only occurs in about a third of cases.

Some readers think Bertha Rochester was suffering the effects of tertiary syphilis, Maybe it was Mr Rochester who gave it to her. If he had, then evidently it was in its latent stage by the time he met Jane. Hopefully, he is one of the lucky ones in which it does not come back.

Carton leads a debauched life, but I still don´t see any evidence in the text that he has got syphylis. He is nail that, for what ever reasons, doesn´t fit the hole, wasn´t made to be sucessfull. Dickens seems to suggest all the time that there is more to Carton than you can see at first sight. His counterpoint in this matter is his fellow lawyer(Strywer I think was the name):strywing, pushing, exploiting, rude, without a grain of sensibility - but successfull, married with children and making a name at the cost of the more talented Carton.

Book 2, chapter 13:

I fear you are not well, Mr Carton."
"No, but the life I lead, Miss Manette, is not conducive to the health. What is to be expected of, or by, such profligates?"
"It is not - forgive me; I have begun the question on my lips - a pity to live no better life?"
"God knows it is a shame!"
"Then why not change it?"
Looking gently at him again, she was surprised and saddened to see that there were tears in his eyes. There were tears in his voice too, as he answered:
"It is too late for that. I shall never be better than I am. I shall sink lower and be worse."...

"Don't be afraid to hear me. Don't shrink from anything I say. I am like one who died young. All my life might have been."...

"If it had been possible, Miss Manette, that you could have returned the love of the man you see before you - self-flung away, wasted, drunken, poor creature of misuse as you know him to be - he would have been conscious this day and hour, in spite of his happiness, that he would bring you to misery, bring you to sorrow and repentence, blight you, disgrace you, pull you down with him."

Sydney is awowedly a drunkard and a man that leads a irregular and troubled life. I understand to "blight you" in this context as, making a girl, that had the possibilities of good opportunities, share his general misery, a misery that seems to include poverty.

A Tale of Two Cities was set in the late C18th. In London at that time about one in five women worked in the sex trade. Sydney Carton is a night owl. He spends much of his time in pubs. Did he always avoid the tarts on his way home? If he didn't, there is a good chance he picked up more than he bargained for.

A good text usually sets the imagination of the readers in motion. That is positive in the sense that it shows their involvement with the story

But you are forgetting that Carton, Bertha, Rochester, etc. are basically creations of ink and paper. The only data you have about the characters are those given by the story.
For example one can´t argue that Carton turned out a loser, because of a problem he had with his father figure, because the text doesn´t reveal anything about his father.
Whatever the method one choses for reading the story one has to stick to the text.

I think authors can allude to things without being explicit about them, and quite often they do. But even if they don't deliberately hide depth charges in their text, if it's in there, it's in there. I once read a brilliant essay on why Gwendolen Harleth from Daniel Deronda had been sexually abused by her stepfather. George Elliot could hardly have stated this explicitly, yet it fits. It need not be deliberate. Charlotte Bronte may conceivably have based Bertha Mason on people she had seen or heard about without knowing they had syphilis. When people write autobiographies, they often give things away about themselves they might not have intended. When authors base their stories on their observations and experiences, they may be including more than they know.

To me it seems quite likely Sydney Carton had syphilis or another STD. I don't think he hates himself because he lets Mr Stryver exploit him. I think he could give up alcohol if he had the incentive, if that's what his problem was. He can't cure himself of syphilis though. Besides, it's a cool theory.

I quite agree with you that a text often implies and suggests things and that the authors often give themselves away.
The question in this case would be: how far one can go with one´s interpretation and yet remain faithfull to the original text. I don´t think Dickens would be very happy about the syphilis theory, neither would Charlotte Brönte.
I think for Dickens what really mattered was showing how a professional loser was able to redeem himself though love. It sounds very sentimental but that´s it.

He gave drinking up before he went to Paris because he had to be lucid to execute his plan.