However hot things start out, there are times when the passion lags—and one person in particular isn't happy about it. We tend to blame the issue on our all-consuming jobs, demanding kids or the pace of life in the 21st century. But—surprise!—that's probably not always the core of the dispute. Alexandra Solomon, a clinical psychologist and marriage therapist at The Family Institute at Northwestern University
, says an overlooked issue is the passage of time. "How someone is sexually at 23 is going to be different at 33 and 43," she says. "Our sex drive is always changing, our interests are always changing and our bodies are always changing." In the early part of a relationship, a woman may initiate making love. Ten years later, she doesn't, and her husband may either lie there wishing she would start things up or do so himself. Learning how to work through this conflict early is crucial, "since you'll likely be renegotiating terms throughout a relationship."