Love is a relationship.
If I must have a definition, this seems as good a place to start as any. This definition is loose, the lines are in pencil and an unsharpened, soft lead pencil at that.
There are many kinds of relationships which allow considerable scope for writers to write love. Love, by this very pencilled in definition can mean anything from pornography to God is love. [My apologies for mentioning anything so crass as faith - but see my History of Religion blog].
A 'Giver' and a 'Taker.' This can be love. One submits and one takes that submission. Most often used in pornography I would say, but toying with it can make erotica play with the line between porn and erotica and be quite effective in taking a reader [not to mention the writer!] into the alternate reality of anything written [both fiction and non-fiction present alternate realities]. It can also figure in deep character studies.
Pure Joy. This is the rarest I would say and in today's cynical and jaded world, the most difficult to make palatable to readers. Here two souls, two hearts, two bodies take joy in each other, in a dance that mimics ideas of eternal bliss.
Impure Joy. Two real people with real character flaws and real strengths bonding firstly as friends - friends being those who stay together while recognizing differences and irritations - and then as lovers, both in spirit and sexually. The most fun and satisfying to write, I would say, as it allows for sparks to fly as well as happiness to settle in to both writer and reader.
There are probably others, well certainly there are many, but off I go to my day job!