impulsive

proceeding from natural feeling or impulse without external stimulus

• “an impulsive gesture of affection”

without forethought

• “letting him borrow her car was an impulsive act that she immediately regretted”

having the power of driving or impelling

• “a driving personal ambition”
• “the driving force was his innate enthusiasm”
• “an impulsive force”

determined by chance or impulse or whim rather than by necessity or reason

• “a capricious refusal”
• “authoritarian rulers are frequently capricious”
• “the victim of whimsical persecutions”

characterized by undue haste and lack of thought or deliberation

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