Good chemistry can develop between a love partner and someone with whom you just click. It's sexual when it's a romantic companion. It has a magnetic quality about it, as if there was static or sparks in the air. You get goosebumps or a prickly warm sensation. You feel drawn to this person even though you may not know exactly why yet. Romantic chemistry doesn't need a reason to exist. It is by definition undefinable, but some people describe it as a feeling of oneness, while others say it is like living inside each other's minds. Some call it magic, some say it's science. Either way, it's that feeling you get when you see someone beautiful eyesight, hear their voice for the first time, or when they tell you they love you. It is hard to describe, but many people have had experiences they call "romantic" with friends or family members they didn't find particularly attractive, elderly people, etc. So, romantic chemistry seems to be an innate human trait that is not limited to young lovers. There are several factors that can affect how you feel about someone else including physical attraction, sexual desire, feelings of love, trust, security, etc. The more you know this person, the better you will be able to judge whether the chemistry is there.