It seems to be a far more difficult question than people give it credit for.
There are different kinds of love, appropriate to different kinds of relationship: that between friends, between parent and child, between individual and humanity; self-love; romantic love; love for experiences or inanimate objects; etc. Asking what they all have in common is, I think, equivalent to asking what love fundamentally is. And that, perhaps, essentially has to do with desiring, or desiring plus taking pleasure in, something's happiness and/or flourishing and/or existence.
So, if that's right, then many kinds of love are straightforwardly real. But romantic love seems problematic to me for the following reasons.
First, I'm not sure it deserves its own category. What makes it different from the sort of love that exists between close friends? There's physical intimacy, of course, but as people are eager to observe, a relationship based solely or primarily on sexual attraction isn't "really" based on love; such a relationship is second-rate, degenerate - it's essentially "lust", or "infatuation". Physical intimacy is thus apparently seen as an "added bonus" to romantic relationships; moreover, it's something that comes and goes; sex becomes dull and perfunctory, or in any case, less frequently desired. But, they say, the love remains all the same.
My other worry arises from the notions of commitment and exclusivity, which are also likely to be proposed as answers to the above question (What makes romantic love different from the sort of love that exists between close friends?). Commitment seems to consist in something that might be called both reciprocal self-denial and reciprocal possessiveness. "We're in this for the long haul"; in less figurative language, "I'll suppress any urge to abandon you if you suppress any urge to abandon me." But what "abandonment" means isn't entirely clear. It seems essentially to involve certain discontinued benefits, or the pursuit of these benefits by means of something other than one's partner. And here the benefits are primarily sexual, it seems. Think of cheating, how that can destroy a romantic relationship, how traces of the desire (in the form of sexual attraction to other people) can never be ruled out - and how that is so often taken to necessitate exclusive commitment. Romantic partners are expected to suppress the desire for physical intimacy with other people; to act on this desire is to forfeit or betray one's love.
Now, what exactly is my worry with this? Just that when we're honest, it seems a bit pathological! It seems to impose arbitrary constraints on love ("So long as I?m with you, you are not to act on certain desires of yours, because...well, because!"), and it seems that when these constraints are violated, people readily allow love to turn into resentment, contempt, or even hatred. ("That no-good, cheating bastard!")
But, again, romantic love without this bizarre sexual possessiveness is hard to understand except as friendship - with unrestricted "benefits".
This all makes me suspect that romantic love is, at best, a spurious concept we invent to glorify something that would otherwise be seen (unfairly, perhaps) as more ordinary and mundane - namely love between friends - and at worst, a spurious concept we invent to de-pathologize our sexual possessiveness. Or some combination of the two.
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That said, I tend to want "romantic love" all the same, and my adult life has been almost completely devoid of it. But not all our desires are rational; some are the result of training and suggestion.
I also fear what a successful romantic relationship would naturally turn into in the long run. (Boredom, restlessness, dissatisfaction?)