Why do some people get aggressive after drinking alcohol?

Being a lover or a fighter when drinking all depends on your personality when sober as alcohol makes consumers more impulsive and exaggerates their usual temperament.

Alcohol alters activity in the frontmost part of the brain (the prefrontal cortex), which is involved in decision-making and self-control, and this makes drinkers more impulsive. This can manifest in relatively innocuous ways – such as being more inclined to tell jokes to a stranger – or in more dangerous ways, such as driving fast or reacting to provocation.

The disinhibiting effect of alcohol exaggerates the drinker’s sober personality. So people who are more inclined to anger, and less concerned about future consequences, are more likely to get aggressive when drunk.