When animus and anima meet, the animus draws his sword of power and the anima ejects her poison of illusion and seduction. The outcome need not always be negative, since the two are equally likely to fall in love (a special instance of love at first sight).
Source - C.G. Jung, The Syzygy: Anima and Animus. CW 9ii: Aion: Researches into the Phenomenology of the Self, par. 30.
If both a man and a woman project their positive images onto each other at the same time, we have that seemingly perfect state of relationship known as being in love, a state of mutual fascination. The two then declare that they are "in love with each other" and are firmly convinced that they have now found the ultimate relationship. There is a certain relationship on the conscious level between the ego personalities of the man and the woman. But there is also the powerful attraction between the two of them which is the result of the projected images of the positive anima and animus. But the most powerful factor of all is the attraction via the unconscious. Here it is as though the animus of the woman and the anima of the man have fallen in love with each other, and here is the bind, the powerful pull between them, the source of the magnetism of the being-in-love state. Most of us can probably remember the first time we were in love, and what unexpected and powerful emotions were released. To have the experience of falling in love is to become open to matters of the heart in a wonderful way. It can be the prelude to a valuable expansion of personality and emotional life. It is also an important experience because it brings the sexes together and initiates relationship. Whether this leads to happy or unhappy consequences, life is kept moving in this way. Falling in love is a natural and beautiful experience, and a life that has not known this experience is no doubt impoverished. The fact is, however, that relationships founded exclusively on the being in-love state can never last.
Being in love is a matter for the gods, not for human beings, and when human beings try to claim the prerogative of the gods and live in a state of "in-loveness" (as differentiated from truly loving each other), there is a movement from the unconscious to break it up.
A relationship of being in love simply does not last when put to the test of the reality of a true, human relationship; it can endure only in a fantasy world where the relationship is not tested in the everyday stress of real life. When they live together in everyday human conditions, "John and Mary" soon become real to each other as actual, imperfect human beings. The more real they are to each other as people, the less possible it is for the magical, fascinating images from the unconscious to remain projected on them. Soon the state of being in love fades away, and, worse yet, the same anima and animus who once fell in love with each other may now begin to quarrel.
Source - John A. Sanford, The Invisible Partner: How the Male and Female in Each of Us Affects Our Relationships (Paulist Press 1980), pp. 17-18.
Art: John Albert Walker, 'Bathers'