According to Dr. David Haig, an Organismic and Evolutionary Biologist at Harvard University, there is a Darwinian reason.  It's to prevent you from becoming pregnant again!  Your infant is trying to eliminate any potential competition for your attention by making you too tired for a romantic romp.

Many moms nurse their baby until six months of age, which acts as a natural contraceptive.  Dr. Haig's research found that when babies reach six months old night waking increases, especially in breastfed babies.  According to Haig, "waking at night to suckle is an adaptation of infants to extend their mothers’ lactational amenorrhea, thus delaying the birth of a younger sib and enhancing infant survival. "

So it's not just your lack-of-sleep induced imagination running wild when you think your infant has ulterior motives.  

Read Dr. David Haig's article in The Oxford Journal of Evolution, Medicine & Public Health.