Why do people need to prove their worth to others?

it's a self conscious issue, one value himself on a certain level and wait for everyone else to agree to that level for him to feel confident.  

In human nature loves excellence and distinctive appearance

Because they need a harmonious insertion in society to justify their existence. At the level of self-esteem corresponds the need to experience a sense of accomplishment and achievement as well as respect of others.

Because they want to be the best 

Because they want to position themselves well compared to others

Because they have ambition 

Thanks 

It is kind of show off people use it between them 

Because the idea of the individual himself is characterized by exclusivity, but they are subject to modification impact of environmental and social conditions that surround it, and the other point of view about it. The individual may see itself in a positive way sometimes, and in other negative, but it is generally a quasi-static conception of himself

Worth should be understood rather than proved. Proving means you are forcing them to say you are good enough.

because only for humanity.

agree with above great answers