We all know this heart isn't anatomically correct; perhaps it was modeled after the heart but in actuality, an oval with arms would've been just as apt a representation. Changizi's idea is that the heart represents a sexually aroused, in this case primate, rump which has natural ties to amorous feelings and therefore Valentine's Day. The red of course being the blood engorged area, which naturally swells; sending a loud and clear "come and get it" message. See below.
This is a proposed theory, of course, and don't take it too seriously just because he's a professor and has a blog. I feel sometimes anthropologists, perhaps social scientists at large, get a bad rap for writing a little less formally and a little more casually online. It's a great theory and I applaud the social scientist who let's those academic ties loosen and engages openly with society.
The rump thing is pretty darn convincing. Changizi's ending point is that the heart symbol has been culturally selected over myriad other symbols of love that no doubt have made their appearance in years past. The selective factor here, and it goes without saying this is not with cognitive purpose, is our adeptness as sensing arousal in a partner. For humans, even though we are the only continually receptive primate on the earth, it is nevertheless important to know when to try a little somethin' and when to back off. The more familiar we are with that, the more sex we will have and the more offspring we will produce, as the theory goes. Many other things in our culture represent physiological arousal or at least preceding or ensuing states, serving the same purpose as the rump heart (e.g. red lipstick).
Bottom line (pun intended), there aren't a lot of explanations for the ambiguous "heart" shape we so love and the red rump thing makes pretty good sense. We are a sexual culture, more so than others and less so than some. It would be an interesting undertaking identifying the cultures where the heart symbol is used on the level ours is and those where they have less meaning and then comparatively analyzing the role sex plays in their lives. This study may not get ample funding but with the global fertility rate slowing as it is (excluding many African countries), more research in cultural sexuality may be deemed important.