An attitude is: 

* the way you think and feel about someone or something
* a feeling or way of thinking that affects a person’s behaviour
*  a way of thinking and behaving that people regard as unfriendly, rude, etc.


Quote: “When you put two negative attitudes together, you will get a reaction of conflicts as an out come”.

On a psychological level:

An attitude is a state of mindset which holds a negative or positive opinion about someone, during the time when that state of mind (or opinion) is confronted, that is when an attitude of reaction (negative behaviour) or action (positive behaviour) is exposed.

a :  a mental position with regard to a fact or state <a helpful attitude>

Why people have attitudes?

Because an attitude is a mental state, that is why negative attitudes can go an noticed due to the fact that most of the people do not have any conscious awareness of their own negative attitudes.

Why people defend their negative attitudes?

Some people know the unpleasant state their negative attitudes can cause, but they deliberately refuse to change them, because they think it is a way to defend, protect and make a point via their own opinion and character.

 
b :  a feeling or emotion toward a fact or state. 

:  an organismic state of readiness to respond in a characteristic way towards a stimulus or stimuli.

The position of an aircraft or spacecraft determined by the relationship between its axes and a reference datum (as the horizon or a particular star)

That boy has a real attitude problem (= behaves in a way that makes it difficult for other people to have a relationship with him or work with him)

​A feeling or opinion about something or someone, or a way of behaving that is caused by this: 

It’s often very difficult to change people’s attitudes.

She takes the attitude that children should be allowed to learn at their own pace.
He has a very bad attitude to/towards work.

He seems to have undergone a change in/of attitude recently, and has become much more cooperative.
I don’t like your attitude (= the way you are behaving).