When the hero's former lover/mate/wife died, making him leery of falling in love with the heroine. He doesn't want to suffer the pangs of love and/or loss and/or betrayal again.
The Regency era was particularly limiting on women's rights and critical of their faculties, which makes the female in disguise plots more harrowing.
When the hero or heroine's financial motivations for marriage (in Regency romance) nearly destroy their chance at true love.
A list of tropes to excuse basically estranged characters getting married in Regency romances.
A heroine in disguise sleeps in the same room as her hero, who thinks she's a man. Also, when the hero discovers her true gender, he walks in on her naked.
When the hero sees, often first meets, the heroine on what he assumes is a runaway horse, but upon closer discovery or interference, discovers she is an adept horsewoman, which he finds very attractive.