Evolutionarily speaking, why do humans have 46 chromosomes
I think the OP is asking why we have one less chromosome pair than chimpanzees (for example) [23 pairs instead of 24].

The is an abundance of evidence, as alluded to above by shigeta, that human chromosome 2 is the result of a telomere-to-telomere fusion of two ancestral chromosomes (IJdo et al., 1991).  This event did not occur in our closest ancestors, hence we have one less chromosome pair.  In fact the sequence of human chromosome 2 contains the relic of an ancestral telomere-telomere fusion (IJdo et al., 1991).

The pdf of this key reference is freely available to all from PNAS 

Reference

IJdo, J.W, Baldini, A, Ward, D.C, Reeders, S.T, Wells, R.A. (1991) Origin of human chromosome 2: an ancestral telomere-telomere fusion  Proc Natl Acad Sci U S A., 88 9051-9055.[pdf]

Actually it has now been shown that Neanderthals and Denisovans also exhibit the same chromosomal fusion as humans - http://m.motherjones.com/politics/2014/02/evolution-creationism-bonobos-neanderthals-denisovans-chromosome-two