I know the symptoms.

He looked almost as sheepish last night as you used to before you proposed to Miss Oleander." It was quite true; the handsome young artist had followed Miss Dane to Washington.

He had hardly known how much he was in love with her until she was gone, and all young-ladydom grew flat, stale, and insipid as dish-water.
Mr.Ingelow, of rather an indolent temperament, disposed to take things easy and let the world slide, was astonished himself at the sudden heat and ardor this little girl with the sunny smile had created within him.
"It isn't her beauty," thought the handsome artist, "although she is pretty as an angel; it isn't her blue eyes and her golden hair, for I see blue eyes and golden hair every day of my life, and never give them a second thought; it isn't her singing or dancing, for half the girls I know sing and dance as well; and it can't be her spirited style of conversation, for that's not so very new, either.

Then what is it ?" Mr.Ingelow, at this point, always fell into such a morass of pros and cons that his brain grew dazed, and he gave the problem up altogether.
But the great, incontrovertible fact remained--he was headlong in love with Mollie, and had followed her to Washington expressly to tell her so.
"For if I wait, and she returns to New York," mused Mr.Ingelow, "I will have Oleander and Sardonyx both neck and neck in the race.

Here there is a fair field and no favor, and here I will try my luck." But Mr.Ingelow was mistaken, for here in his "fair field" appeared the most formidable rival he could possibly have had--a rival who seemed likely to eclipse himself and Oleander and Sardonyx at one fell swoop.
At the presidential levees, on public promenades and drives, Miss Dane had noticed a tall, white-haired, aristocratic-looking gentleman attentively watching her as if fascinated.