Tuberculosis, or consumption as it was commonly called, was definitely romanticized for a good portion of the 19th century. The more obvious reason were the physical symptoms of the disease: flushed cheeks, pale and translucent skin, dilated pupils. "The bright crimson spot which burnt in each cheek, and the vivid fire of her eye when roused up by fever or excitement, gave her fair and beautiful face an expression of almost angelic loveliness.” The Magnolia 1, no. 1 (Jul 1842): 37.

Consumption more commonly afflicted women, as well, and was considered a disease of the fragile and vulnerable. At its peak in the early half of the 19th c, the system of transmission was not understood. Some fell ill easily, while others in the same house never caught it. So it fell to the idea of predisposition, which played on the physical, mental, and emotional state of the person. They focused on prevention, since there was no cure, often based around the assumption that the physical appearance of a person was an early warning sign. "But though those predisposed to consumption are often as beautiful as the flowers of spring, they are delicate and fragile. They usually have slender forms and narrow chests; their lungs are easily irritated..." The Knickerbocker July 1836

The second part to this is the more complex view of religion and suffering in that era. To endure suffering was seen as a method of atonement and a way to achieve purity. So, dying a slow and difficult death, especially when the person looked "angelic" was spun in a positive and very romantic way. A great example of this "suffering produces morality and purity" concept come from stories like The Water-Babies (1863): a popular 19th c fable where a child dies but becomes an insect for a time to atone and learn morality before he can reach heaven. The Little Match Girl (1845) is more well known today, but has a similar theme of romanticizing suffering.

Many of the stories that discuss the disease from a fictional stand point use incredibly flowery language to describe the entire process. My particular favorite is that of "Agnes Waltham" published in 1842. She falls ill after walking outside in thin shoes one evening, much to the frustration of her parents and fiance. She is taken to warmer Cuba for health, but returns home near death where her fiance sees her for the first time in months: "Oh! the sight that met his eyes. Pale, and worn to a skeleton, yet with the lustrous eye and crimson cheek of the consumption, Agnes Waltham met the eye of her lover, who had parted with her, when she wore at least the appearance of health. The change was too much for him, he staggered to a chair, and for some minutes could not speak. Her parents wept aloud." Agnes asks to be forgiven her sin of vanity and the fatal pride of dress, then smiles “an angel’s smile” and passes away.

Most stories like this, or even nonfictional descriptions, account that the person was in a dreamy, calm state at the end. Often, they ask for forgiveness or some other form of penance is expressed before they pass away silently. The calm and barely lucid state at the end is medically accurate, though the rest is left up to the readers to believe I suppose. All in all, there is a lot that goes into making Tuberculosis a romantic disease in the 19th c. Most of it comes down to the fact that they didn't know what caused it or how to treat it. So they grasped for any and every reason they could find and sort of stereotyped the disease and its victims to give a sense of clarity as to why so many young and seemingly innocent people were dying (not just the poor and unhealthy). I highly recommend "Consumptive Chic" by Carolyn A. Day for further reading on the topic.