It is a real thing. You can find more detailed explanation on Married Man Sex Life blog, but essentially there are three things people can mean when talking about "love", and each of them is driven by a different set of chemicals.

a) obsession
b) closeness
c) sexual desire

The obsessive love is A + C, or sometimes just A. The mature love is B + C, with a smaller intensity of A returning shortly once in a while, usually when you break your stereotype in a good way, e.g. go together on an exotic vacation, or go dancing.

You probably already have the experience of B without C. It's what you feel towards good friends (the ones you feel safe with), family members, or perhaps your pet. And what you will later feel towards your children. The feeling increases if you touch someone in a non-sexual way (e.g. hug, or dance with), or if you look deeply in their eyes (assuming you already feel safe with them).

Now imagine this, in a high intensity (but only when you think about the given person, usually when you are with them, not obsessively all the time), with the sexual undertone. (The sexual undertone doesn't have to be there all the time; however its long-term absence is a frequent failure mode.)

To protect this mutual feeling: Act in a safe way; don't ever hurt the other person physically or verbally. Express your love verbally (no need to be dramatic, just make frequent casual nice remarks) and non-verbally (short but frequent affectionate touches and hugs). In nice circumstances, look each other in the eyes. Be sexual once in a while, but not all the time (send a message that both modes are great with you). -- Read this book about individual differences, because I described what works in my relationship.

Typical mistakes: Acting safe doesn't mean you should stop joking and teasing. Actually, you shouldn't; you just need to calibrate and stop immediately when it becomes inconvenient for the other person. Just because you feel safe with the other person, don't make them your psychologist. It's okay to share some of your problems, but don't put the whole burden on a single person, just because the person is there and it's convenient for you. (Specifically for men in heterosexual relationships: make your partner feel safe and loved even when she is not in a mood for sex; but don't let too many days in a row pass asexually, because her mind can switch to "I love him, but I'm no longer in love with him" and suddenly you are friend-zoned and/or waiting for a divorce. Sex is part of the package, even if it doesn't include penetration.)

There is a role of deciding here -- you have to decide to act in a way that allows this feeling to develop, and to refrain from acting in a way that would destroy it. But the feeling is emotional, chemical, you feel it, not just tell yourself that you have it. Maybe for people coming from healthy loving families the decision component is invisible, because that's how they behave naturally. If you were less lucky, you have to pay attention. It's probably good do understand it explicitly, anyway, to prevent a random screwup. Also, this is how your partner should behave, too... if they don't, tell them... if they don't bother to listen, unfortunately I have a bad news for you. The good news is there are other people out there. Sometimes people need time to learn, but usually that also includes a change of partner.