Mad Jack in Love

In this case it was unrequited love. Jack Fuller made a proposal of marriage to Susannah Arabella Thrale in 1790, when he was 33, and was rebuffed. He never married.
Jack took it hard and tried to torment Susannah, then 20, for her refusal. In 1790 a Mrs Henrietta Henckell Hare wrote to a friend that:
'Our neighbour at Rose-Hill has been lately refused by Miss Susan Thrale. He is so angry with her that he has brought down a woman of the town to Tunbridge Wells on purpose to distress her by following her about everywhere. If this is the fact I think him a great fool.'
In 1781 Fuller had been a favourite of Susannah's elder sister Hester (Queenie) but mother did not approve.
Mad Jack had much to commend him as a young man - he was very wealthy and not unpleasing to look at - in 1779 Fanny Burney described him thus: "He is a Young man of a very large Fortune, remarkably handsome, and very gay, sensible, unaffected and agreeable."
He was not without suitors. Fanny Burney tells us "Peggy Pitches, who is the greatest little Coquet in sussex, fixed her Eyes, and aimed her dart, at Captain Fuller, - she smiled, tittered, lisped, languished, and played pretty all the Evening, - but the Captain was totally insensible, -he has, indeed, so little passion for flirtation...."
Surely John Fuller would have been aware that he was something of a catch and that he would attract attention for all the wrong motives.

Sources:
"John Fuller Esquire of Rose-Hill" - James Lawrie
"Journals and Letters" - Frances Burney
"Fuller of Sussex, A Georgian Squire" - Geoff Hutchinson