Love is when you have a relationship with someone which causes you to care more about them than you do about yourself. It exists because we need to have kids and raise those kids to survive as a species. To encourage this, our biology enables us to love. We (often, but not always) fall in love with the mother (or father, if you're female) of our children and stay with them at least long enough to get the kids out on their own. All the stuff associated with romantic love - passion, physical attraction, dedication, courting, etc - stems from and facilitates this basic biological need. Similarly, we love our children because we are supposed to raise them, so there needs to be some biological imperative to keep them around for 18+ years. I mean, seriously, kids are a pain in the ass. If people didn't love their children, would anyone voluntarily keep one or more of them [i]in their goddamn house[/i] for upwards of two decades? Hell no. They require a massive expenditure of effort, time, and money and they require you to prioritize their needs over yours. People aren't exaggerating when they say that having kids changes your life forever. It absolutely does. We also love the members of our nuclear family (parents, siblings) for similar reasons. Family unity is important for both social and economic reasons. It provides a support structure that fosters education, financial stability, and social integration, which all tends to create more well-balanced individuals who have a better chance of success at procreation. Obviously, none of those things happen 100% of the time. Some people don't love their kids. Some people don't love their dad or whatever. But hey, guess what, some people are psychopaths too. We have serial killers, terrorists, suicide bombers... Ours is a uniquely self-destructive species. Our capacity for abstract thought can interfere with our biological tendencies and get in the way of normal human development. Plus, there are people who unfortunately just make a bad roll of the dice and end up with shitty parents, abusive spouses, etc... "Is it merely an emotion activated by certain stimuli that cause the neurotransmitters in the brain to create this intense feeling? In this sense, would that mean love is a programmable, predictable thing?" All emotions are activated by stimuli that affect your neurotransmitters. Does that make them any less powerful or genuine? "Is physical attraction necessary for love?" No. You love your children, you love your parents, you love your siblings. If you are talking specifically about romantic love, it's not necessary there either, but it does help enormously. That makes sense from a biological standpoint too. Relationships are hard work and they sap a lot of your freedom away from you, so you're more likely to commit to something like that if she's pretty and she's got a nice ass. "If you believe love is evolutionary, as in an emotion forged by the brain to influence reproduction, how does love exist between homosexuals?" It doesn't work the way it should every time. Homosexuality exists in other species, but the human capacity for abstract thought creates an even greater deviation from the norm than you see in animal species. There are theories that homosexuals serve a greater purpose in society too. For example, in times of high birth rates, having a large number of homosexuals can stabilize the population. In addition, many homosexuals go on to work in creative fields, which contributes to technological innovation and social progress. (Remember, "technology" isn't just computers, it's any tool that we use to improve the quality of our lives.) "Have you ever thought you loved someone, and then realized you didn't actually, it was just a spur of the moment emotion?" Yes, in high school, but I think this happens to a lot of people. Like anything in life, you need to ride with the training wheels on for a while before you master a skill. Loving is a skill; it takes work, commitment, maturity, and personal sacrifice on both sides to maintain. When you're young, you still need to develop those skills. Thus, play dating, puppy love, etc...