This is a question which attracts many theories and opinions from well-meaning scholars, doctors, and therapists, as well as 'the man (or woman) on the street. In fact, we are all subject to the affects of what we call 'Love' in one form or another. But, as The Buddha is reported to have said 'People with opinions just go around bothering one another!', so I am not going to offer you another opinion... I know what love is...
(This subject is explored / described in great detail in our books and CDs, but here is a little to get us started...)
Many believe, and some have had their own insights so 'know', that everything here is connected. As a Self-Realised person I say I know more about the nature of reality than most, but only because I have realised it. The same truth is within everybody, so I am not special.
The name Self-Realisation is interchangeable with other names, including God-Realisation, and this is because they are one and the same. To realise one's self (or, 'oneself') is to realise God. But do not imagine I am referring to a God who is separate from you, for that is not the truth. There is one Being here, and it is the space you feel inside you now. That is the same space that exists within every living thing, in every object, and is the nature of this existence. That space is God.
Now, because we have forgotten this truth, we feel separate from everything, even from our own self much of the time. But every so often someone (or something) reminds us and reconnects us with a part of ourselves and we feel complete. It is this that we call Love; this new feeling of being 'whole'. When the person goes away, we feel incomplete, and that is why love hurts. In truth you can see that it is not the love that is hurting, but the absence that we now feel, and the task at hand is to learn to connect with the love all the time, so you are always complete and never lose that feeling of completeness or being whole that is called love. When completed, this task is what is called Self-Realisation or God-Realisation, and is also why the great Spiritual Masters have been described as being in a state of love all the time.
So, What is Love? Love is connecting with your own self, your own being, so you are conscious and thus are complete.
And this is of course quite different from the 'emotional love' that many of us are familiar with, and which is often regarded as having a fine line between it and hate. There is no fine line between love and hate. There is only consciousnesses or unconsciousness.