"an impulsive gesture of affection"

"letting him borrow her car was an impulsive act that she immediately regretted"

"a driving personal ambition"
"the driving force was his innate enthusiasm"
"an impulsive force"

"a capricious refusal"
"authoritarian rulers are frequently capricious"
"the victim of whimsical persecutions"

"a hotheaded decision"
"liable to such impulsive acts as hugging strangers"
"an impetuous display of spending and gambling"
"madcap escapades"