Standing in love differs because unlike falling in love, which is premised on the fact that the lovers are still more or less strangers to each other, standing in love is to love a person because they are as well known to you as you are to yourself (or not as the case may be). Falling in love becomes standing in love, if it does, when the thrill of the unknown becomes the delight of knowing another and being known by them.
The difference comes out, I think, when you think of the difference between the exclusivity that people falling in love want from each other and that of the exclusivity which exists between people who are standing in love.
When you fall in love, you want your partner to be faithful to you because if they are not it threatens your loneliness again. They might leave you, and leave you alone. This is possessiveness. So it is quite possible to find two people who are apparently in love with each other and who actually feel no love for anybody else. These are the kind of lovers who are completely annoying to be with. They are so involved with each other that they do not notice the rest of the world. They make you feel alone when you are with them. They think of love as luck and that their luck is in ??” and conversely, that everyone elses luck is not.
There luck is not in, though, because their love is, in fact, what Fromm calls an egoism together; they are two people who identify themselves with each other, and who solve the problem of separateness by enlarging the single individual into two. It is in fact narcissism ??” they love themselves in each other; they see each other as Narcissus starred into the lake. They have the experience of overcoming aloneness, yet, they are separated from the rest of humankind ??” which is why you feel lonely or annoyed in their presence. In fact, they too remain separated from each other and alienated from themselves, though they darent admit it and so become even more absorbed in each other. Their experience of union is an illusion.
When you stand in love, though, you want your partner to be faithful to you but not because you cannot be alone but because it represents to you the faithfulness that must exist between all human beings who are to relate well to each other. In other words, it is not an exclusive possessiveness but an expression of an inclusive love for all humankind, potentially at least. Thus, the nicest people to know who are in love with each other are those who make you feel part of their love, whose love generates a welcoming home, brings out the best in you and so on. They have learnt the art of love with each other and it results in generating love that they have for others.
Another feature of this love as an art is that it makes it essentially an act of will, of decision to commit my life to that of one other person. Hence the faithfulness again. This feels completely the opposite of love when it is understood as spontaneous, emotional and sudden. Also, it suggests that to love someone is not just to have a strong feeling. You may often have no strong feelings at all some of the time when you stand in love. Rather, love is better understood as a judgment or a promise. If love were only a feeling, there would be no basis for your promise to love someone. For a feeling comes and goes. You cannot promise that.