As part of Big Think's "What Is Love?" series, we release interviews with Helen Fisher and David Schnarch, two notable experts in the field of love. According to Fisher, there's a reason that men and women in relationships often find it difficult to engage in deep conversations. Could it be something that dates back to our early evolution?

Fisher also talks about what makes us attracted to some people and not others. For example, why were Bill and Hilary Clinton so compatible in the first place? Fisher says it has a lot to do with Bill's emotional side.

David Schnarch talks about the psychological effects of Valentine's Day on relationships, and offers tips on how to spice up a 25-year-old marriage. The best thing to do? Lie next to your partner and engage in mutual gazing. Schnarch explains the science behind what seems like a silly idea