Why are Chihuahuas so aggressive and confrontational, despite their small size? From an evolutionary standpoint, wouldn't this be considered disadvantageous and be naturally selected against as it's provoking unnecessary conflicts?

Chihuahuas have different brains than all other dogs, but sadly their owners are not educated about this. This results in the dogs not being handled and trained correctly and becoming fearful little bullies.

The brain of the chihuahua is neoterized and does not mature like a dog’s.
A wolf brain goes through 5 stages of maturity. A domestic dog goes through 4 levels and stops (thus, tail wagging continues all their lives).

So, Why are Chihuahuas so Aggressive?

A chihuahua’s brain only goes through 3 levels and stops, so they are like permanent puppies, and more immature. This means they require more patience, consistency, and firm structure than the average dog. They should not be carried around and treated like dolls, or isolated from other dogs and people, especially early in life.

Domestic dogs did not “evolve”. Humans selectively bred them, and humans are foolish and bred chihuahuas for size, not temperament or intelligence or good health. The Mexican chihuahua is a larger type, often more calm and well-adjusted. The tinier the dog, the more problems they have.

Carefully socialized chihuahuas who have been treated like dogs and allowed to develop their dog body language and coping skills can be calm, happy, and relaxed, but it is rare.

It is never too late to help a chihuahua who is a fear biter, or unsocialized. but it takes time and patience. Many, many of these little dogs end up in shelters and are destroyed when owners cannot handle them and get a new baby or someone moves into the home, or an owner dies and no one wants to deal with a dog’s issues.

Source: quora