I've been in two serious relationships and loved both people very deeply. There's been a handful of other crushes and shorter relationships, but none of them especially serious.
The first of those was when I was 16 and lasted until I was 18 or so and made messy by religion on both sides (Combine repression of sexuality with raging hormones on both sides, but dominantly mine. You get the picture.). We met in church when I was practising the organ, got chatting, we got quite close and then ended up going out 6 months later. It lasted about a year and a half in total and eventually disintegrated for a lot of different reasons.
The second relationship I'm currently in and have been for over a year now and are looking towards getting a place together in London next year once I graduate. We met at uni and got together pretty soon after we got to know eachother properly and fell head over heels for eachother within the space of a few weeks.
As for love as a general concept, I don't think it's possible to ever finitely identify your reasons for loving someone, because it's not a rational thing. You could list 10 million reasons why you should or could, or why a relationship would work really well, but if you don't have that intangible spark or attraction, then it just won't happen. You might have reasons for liking someone, but like or lust doesn't always turn to love (And love is quite hugely distinct from infatuation or the falling in love at the beginning of a relationship.). Love is, by my definition, a state of mutual respect and willingness to sacrifice anything and everything for that other - existing as both individuals and a pair intertwined on every possible level, sharing every possible dream, hope and thought.
For interest, and because some others have already mentioned it, I'd like to draw attention to the Greek distinction between different kinds of love (Which exist in harmony rather than mutual exclusion). Eros, Philios, Agape and Storge. Eros is love as being 'in love' (Not necessarily linked with sexuality, however), Philios the bond between friends and like-minded individuals and Storge the love shared between family members. Agape is a different thing that has primarily theological significance for Christians in describing their view of God's love for mankind. The first three are sometimes helpful to think about when discussing a concept as complicated as love with so limited a word.