Why is matching function increasing in arguments? In matching theory, the matching function $m_t=m(u_t,v_t)$, where  $m_t$ is the number of matches(new hires) between unemployed looking for work($u_t$) and vacancies($v_t$).

This matching function is usually assumed to be increasing in its arguments, i.e. partial derivatives are positive. 

What's the rationale for this?

Is there any data allowing us to conclude that it should be so?

Any help would be appreciated.
Think about it this way. The function is measuring the number of new matches being created. If we increase either the vacancies or the amount of unemployed, the function will generate more matches. Therefore, the partial derivatives are expected to be positive.

For example consider the matching function with something other than employment. If we consider a marriage game where we have a pool of single men and women, increasing either the amount of single men or the amount of single women, the amount of new matches created should increase. Note, that while increasing one of the two arguments might not actually increase the amount of matches, at worst it has no impact (such as in the edge case where we have only men and add another man). Since our function only considers new matches created in time T, increasing either argument should have a partial derivative of greater than or equal to zero. i.e. creating more matches.

Imagine now that instead of single men we have active job seekers (analogue to unemployment), increasing unemployment causes an increase in new matches.