Humans are attracted to the scent of people genetically different than them.

What this means is that we have a biological tendency not to commit incest because the smell of people genetically similar to us is not as attractive. Instead, someone with very different genetic material and immunities will tend to smell a lot better to us. Scientists found out this connection by having guys sleep in the same shirt for five nights. 

That way their individual smell would be on the shirt. They then had several different girls smell each shirt and rate the smell on attractiveness. Results clearly leaned towards establishing that girls are more attracted to the scent of a guy with more differences in genetics from her own genetic makeup. Evolutionary psychologists have been working hard to establish relationships such as this one and other similar connections rooted in evolutionary theory. While the research is not definitive, they are able to provide evidence for connections that make sense. 

For example, many people are afraid of spiders and few people like the smell of rotten eggs. It makes sense that we should inherently dislike things that may have posed a danger to us now or in the past. 

(Source)