Data, an android on the TV show Star Trek: The Next Generation, defined friendship like this:
"As I experience certain sensory input patterns, my mental pathways become accustomed to them. The inputs eventually are anticipated and even missed when absent."
This is a good definition of friendship, but it's also the definition of love.
People are often attracted to each other, not just physically, but through all sorts of different means. For example, one person might strike up a conversation with another and find they have a lot in common, or they find the same types of things to be humorous. This could turn into a relationship, and they could soon find themselves "in love."
But is love a thing? Is it some chemical coursing through our veins, or a vapor that snakes through the universe, infecting humans?
More likely than not, it's just a word that we assign to our feelings. To those "sensory input patterns" and "mental pathways" we become accustomed to.
Even the word "feelings" is a bit odd, like they are something other than thoughts and sensory inputs. Humans are more like the android Data than we wish to believe, because we think, we touch, we hear and we feed all this information into our brains, and it tells us what we feel.
Our brains "inform" us that we are now "in love."
Not that any of this discounts those feelings. When we have that overwhelming desire to be with someone, to see their face, feel their hand in ours, to hear their voice, and to kiss their lips, it sure feels like love.
It feels like the greatest thing to have ever happened to us.