No man is so entirely masculine that.
Two Essays on Analytical Psychology
Here, without a doubt, is one of the main sources for the feminine quality of the soul. But it does not seem to be the only source.
No man is so entirely masculine that he has nothing feminine in him.
The fact is, rather, that very masculine men have — carefully guarded and hidden—a very soft emotional life, often incorrectly described as ‘ ‘feminine.”
A man counts it a virtue to repress his feminine traits as much as possible, just as a woman, at least until recently, considered it unbecoming to be “mannish.”
The repression of feminine traits and inclinations naturally causes these contrasexual demands to accumulate in the unconscious.
No less naturally, the imago of woman (the soul-image) becomes a receptacle for these demands, which is why a man, in his love-choice, is strongly tempted to win the woman who best corresponds to his own unconscious femininity—a woman, in short, who can unhesitatingly receive the projection of his soul.
Although such a choice is often regarded and felt as altogether ideal, it may turn out that the man has manifestly married his own worst weakness.
This would explain some highly remarkable conjunctions. ~Carl Jung, CW 7, Para 297