impulsive

Determined by chance or impulse or whim rather than by necessity or reason.

Having the power of driving or impelling.

“an impulsive force”

Characterized by undue haste and lack of thought or deliberation.

“liable to such impulsive acts as hugging strangers”

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”