There is a constant battle between the genes of males and females in a species about what is most adaptive. This is an interesting article on how it occurs in humans. http://mikemagee.wordpress.com/2010/04/01/patriarchal-society-and-the-biological-battle-of-the-sexes/

The particulars of giraffes, I don't know. All I know is that the eating leaves from trees hypothesis didn't hold up and a simple google search could explain why.

Edit:

There are two main hypotheses regarding the evolutionary origin and maintenance of elongation in giraffe necks.[36] The “competing browsers hypothesis” was originally suggested by Charles Darwin and only challenged recently. It suggests that competitive pressure from smaller browsers, such as kudu, steenbok, and impala, drove the elongation of the neck so giraffes could reach nutrients competitors could not. This advantage is real – giraffes can and do feed up to 5 m, while most of their competitors, kudu, can only feed up to about 2 m (7 ft).[37] There is also research suggesting that browsing competition below 2 m is intense, and giraffes feed more efficiently (gaining more leaf biomass per bite) higher in the canopy.[38][39] However, scientists disagree about just how much time giraffes spend feeding at levels unreachable to other browsers.[36][37][40] Although giraffes can feed as low as 0.5 m and as high as 6 m off the ground, it appears that they most often feed between 2 and 4 m (7–14 ft).[40] However, elephants also routinely feed at heights up to 5 m (they knock down only a minority of the trees they feed on), and are likely competitors at these heights. Competition for food with other giraffes could also favor the evolution of tall necks.

The other main theory, the sexual selection hypothesis, proposes that the long necks evolved as a secondary sexual characteristic, giving males an advantage in "necking" contests (see below) to establish dominance and obtain access to sexually receptive females.[41] In support of this theory, males have proportionally larger necks than females,[36][41] and males with longer, bigger necks are more successful in dominance displays and courtship behavior.[42] However, a major criticism of this theory is that it fails to adequately explain why female giraffes also have long necks.[43]

Congrats you found the major criticism