I have a hypothesis that the sensation we describe as love is actually a feeling of relative safety that another individual provides. The "relative" part is important. Allow me to expand on this idea.
Humans are essentially animals that somehow learned to read. Our most basic instincts for survival are still very much intact. We are living danger detectors. That feeling of stress you experience so often is your early warning system.
Danger can come in many exotic forms. One form of danger involves physical safety, and here we can see that our loved ones would be the most reliable when it comes to protecting us in a dangerous situation. When the bear goes after you in the forest, your coworker tries to outrun you, but your spouse is likely to grab a tree branch and join the fight.
You will be tempted to argue that an armed hunter who happens to be in the general area during the bear attack would be more beloved than your spouse, according to my hypothesis, because only he can shoot the bear and revive your feelings of safety. But I think the hunter example supports my argument. If the hunter kills the bear and saves your life, you will in fact feel an immediate and deep affection for him that is a lot like love. For cultural reasons, you won't define your feeling as love, but it will feel spookily similar. And you know that after you thank the hunter, he will no longer be your protector. Your feelings for him are temporary.
Likewise, there's probably a good reason that women are often attracted to men in uniform, particularly the ones in lifesaving professions, such as police, military, and firefighters. As further evidence for my hypothesis, a cool uniform doesn't benefit doormen or waiters in the love department. For women, it must be the feeling of safety that makes a difference. It probably also helps that healthy-lookin men are more likely to produce healthy babies, which in itself makes a mother safer as the children get older and can help out.
Danger comes in many forms beyond physical peril. For example, one of my worst fears involves the risk of loneliness, or the risk of not being seen as useful to others. For me, that would be worse than death. Our loved ones are the best protection from that sort of danger. As long as you have a good relationship with your family, significant other, and friends, you feel safe from the dangers of loneliness. And you always feel potentially useful.
From a species perspective, our fear of eventual death is closely related to our impulse to spread our genes and create a sort of immortality. We feel love for the person we see as baby-making material, even if we override the instinct for reproduction for practical reasons, such as economics, age, etc. We're simply wired to feel safer, gene-wise, when we're around someone who might help us reproduce.
Religion also supports my hypothesis. The pious don't simply prefer God, or find it convenient to obey God. They literally love God. This is consistent with my hypothesis because the opportunity for an afterlife is the ultimate safety net. Even if things go pear shaped during life, believers still feel safe in the long run, and therefore they feel love.
Your dog appears to love you above all others, but it's no accident that you are your dog's main protector. You feed it, shield it from bigger dogs, shelter it, and let it sleep near you at night for group protection. In return, you know your dog will make you feel less lonely. We're a species that relies on group size to keep us safe. The more creatures we have on our side, the less likely we will be attacked.
A cat is harder to explain by my hypothesis. A cat makes you feel less lonely, but it has little or no protective qualities beyond sensing approaching danger faster than you can. I think that explains why an unusual number of men dislike cats: Cats don't have your back when the trouble comes down.
Love has many flavors, of course. You experience different kinds of love for a spouse, a family member, a friend, a pet, a hero, and a deity. My hypothesis is that each of those flavors of love is related to how safe each individual makes you feel. A little bit safe feels different from very safe.
That's my moist robot explanation of love. I hope I didn't ruin it for you, or minimize its importance. Making another person feel safe is the most perfect gift you can give. Love is the glue that binds society. If my hypothesis is correct, love is how you know you're doing things right.