Some people believe in love at first sight -- that immediate jolt of electricity that signals powerful attraction and instant connection to someone you've just met. It's sometimes accompanied by a sense of recognition, such as a feeling that you knew each other in a previous life. However, for this initial surge of chemistry to develop into long-lasting love, it's necessary to deepen the relationship beyond the desire for sexual activity. Over time, as the couple communicates with each other, confide in, listen to and support each other, the love can grow and withstand the test of time.

Two people who feel chemistry for each other might explain the attraction as based upon the commonalities between them. Relationship expert Dr. Laura Berman says we're often attracted to people who look similar to us because we're innately drawn to the familiar. We're also more likely to feel relaxed and comfortable with those who share our backgrounds and worldviews. However, chemistry can also exist between unlike persons. Helen Fisher, PhD, author of, "Why Him? Why Her?" suggests that chemistry can occur when people recognize that their traits complement each other.

Scientific research reported in The Economist explored the role biology plays in the attraction process. Animal studies found a connection between the release of brain chemicals such as dopamine, vasopressin and oxytocin and sexual attraction. Research has also found that natural hormones and birth control influence attraction. The Wall Street Journal reports research findings in which men were more attracted to women during the fertile phase of their ovulation cycle, probably due to pheromones, the subtle smells that people exude.

Although there are some differences between how men and women experience the chemistry of attraction, there are some surprising similarities. Dr. Helen Fisher found that over 80 percent of both men and women regard talking about their relationship as an important way to fuel their attraction. Science Daily reports that physical beauty is important to both genders, as is earning power. Other factors, including kindness and a sense of humor, also ranked highly with both genders as contributing to the chemistry between two people.