The Connection Between Fitness and Singles

What's the connection between dating and singles? It seems like gyms are often popular places to meet people, and fitness singles sites are cropping up to satisfy the needs and wants of online daters looking for partners to workout with and romance at the same time. Is there a significant connection between the fitness industry and single life?
One possible explanation for the connections made between fellow gym-goers is the desire to find like minded people who enjoy active lifestyles and staying in shape. Certainly, a part of this is a greater appreciation in general for people who are physically fit. While this is a broad generalization, it may be safe to assume that most people who want to keep themselves in shape also want that from their partner. For this reason, gyms attract like minded people, with similar lifestyles. Also, as has been proven by several psychological studies, similarity and proximity increase the likelihood of attraction. Therefore, if two people run into each other on a regular basis, and see each other participating in activities they both enjoy, the chances of meeting and making a connection are more likely.
Another psychological explanation may come from biological and neurological studies. Chemicals released into the brain during exercise are often associated with feelings of euphoria. These feelings are commonly seen as responsible for a 'runners high' that many consistent exercisers experience. As these feelings of euphoria are experienced, people are conditioned to associate those positive feelings with other stimulus that surround them. Like Pavlov's Classically conditioned dogs, a person who see something (like another attractive person) while experiencing a positive emotion (a runners high) begin associating these two things together. Eventually, people begin attributing their positive feelings to that person (he/she makes me feel good). As a result, many singles who are heavily into the fitness lifestyle, and regularly visit the gym, may begin to pair their positive feelings with other fit singles they see at their gym.
While these explanations may only be a part of the reason why singles and fitness are frequently paired together, ultimately, there is some sort of connection between the two. Whether these stem from psychological reasons or the desire to seek out other physically attractive or fit people, fitness and singles are frequently paired, and with good reason.