Lawrence Salcido said:
When you trust her enough to play your Skyrim account.
That's love.
Or your Diablo/Dark Souls/any-long-investment-RPG account.
OT: Not the slightest clue. I'm going to assume I've never been 'in love' (I'd probably know if I was), but I think it is just a vague term people use when they have an intense emotional/physical/social attraction to someone, and it can manifest as selfishness (Jealousy, possessiveness), selflessness (putting them first), shared experience (you work together for whatever benefits 'the couple' the most), or some combination thereof.
Also sex, people seem to like having sex with people they care about.
But yeah, it seems to be somewhat vague, and even when the experience is the same, the responses it provokes are different between individuals and couples.