In other words, you can only want something that you don't have; if you already had it, there would be no point in desiring it. At the same time, you have to be aware that you are lacking it: if you didn't, you couldn't have a desire either. A good example is love of wisdom: you can only be a lover of wisdom if you are not wise yourself, for if you had wisdom already, there wouldn't be a need to strive for it any more; but at the same time, you cannot be a lover of wisdom as long as you are totally ignorant: because then you wouldn't know anything of such a thing as wisdom, and hence you couldn't desire it. For love to be in play, there must be something you lack and therefore desire, something that you want, but know you do not have.
Love implies, then, that there is an instance of unreality in play. In order to form the desire that is the basis of love, we need to form an idea of something that is not the case (but which we desire to be the case). It's not love if there isn't a lack, and we must come up with an idea of that lack; it's unreality, because we're thinking of something that isn't so (yet), and we must produce it (it wouldn't be unreality if we didn't produce it). We must be both lacking something and be aware of the lack.
(This is the formulation I would give it in the terms I've been using in this blog; for reasons that will emerge soon, Plato would certainly be suspicious, to say the least, of that way of putting it.)
 Symp 200a–e. We should keep in mind that, throughout this discussion, the focus is on eros, which is only one of the various forms of love. Both in ancient Greek and modern thinking there's much complexity to the concept of love; we might distinguish different forms to the extent that it could even appear that there really is a multitude of different concepts behind the single word. But I'm not going into that variety here.
 Symp 204a
 Symp 200a–b. Moreover, Socrates doesn't just get Agathon (his partner in the discussion) to confirm this stronger claim (of a strictly conceptual necessity), but he also discusses an obvious objection. Sometimes it might seem that a person has a for something that he already has; but this, Socrates argues, cannot really be a desire for getting that thing; instead, what the desire really points to is a future continuation of these attributes: you desire to keep that thing which you already have (200b–d).